# Révision du consensus sur l'effet de l'acidification des océans sur la calcification des coraux

Riccardo Rodolfo-Metalpa

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To cite this version:

Riccardo Rodolfo-Metalpa. Révision du consensus sur l'effet de l'acidification des océans sur la calcification des coraux. Cellular Biology. Université de la Nouvelle-Calédonie, 2024. tel-04621346

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Distributed under a Creative Commons Attribution 4.0 International License
Université de la Nouvelle-Calédonie – Ecole doctorale du
Pacifique

Habilitation à Diriger des Recherches

UNIVERSITÉ
de la
NOUVELLE-CALÉDONIE

ENTROPIE
École Nationale d'Études des Risques, de la Santé et de l'Industrie

<figure><img src="image_3.png" /></figure>

Institut de Recherche
pour le Développement
FRANCE
French National Research Institute for Sustainable Development

Titre

Revising the consensus on the effect of
ocean acidification on coral calcification

Révision du consensus sur l'effet de
l'acidification des océans sur la calcification
des coraux

Dr Riccardo RODOLFO-METALPA

Chargé de recherche

UMR ENTROPIE

Institut de Recherche pour le Développement

Nouméa, Nouvelle-Calédonie

Soutenance prévue le 20 Février 2024

Jury :

Yves Letourneur (Professeur, Université de la Nouvelle-Calédonie)

Président

Sophie Cravatte (Directrice de Recherche, UMR Legos, IRD, France)

Rapporteuse

Luisa Mangialaio (Professeure, Ecoseas, Université Côte d'Azur, France)

Rapporteuse

Matthieu Pernice (Directeur, Professeur associé, UTS, Australie)

Rapporteur

Cyril Marchand (Professeur, Université de la Nouvelle-Calédonie)

Examinateur

Nazha Selmaoui-Folcher (Professeure, Université de la Nouvelle-Calédonie)

Examinatrice
TABLE OF CONTENTS

<table><tr><td>1. Curriculum vitae</td><td>4</td></tr><tr><td>2. Preambule on my research activities, projects and research history</td><td>19</td></tr><tr><td>2.1. Contribution to my field</td><td>19</td></tr><tr><td>2.2. Recognitions from others</td><td>19</td></tr><tr><td>2.3. Research history</td><td>21</td></tr><tr><td>3. Why should corals care about ocean acidification: toward a revised consensus?</td><td>23</td></tr><tr><td>3.1. Building the consensus on the ocean acidification threat</td><td>23</td></tr><tr><td>3.2. Effect on calcification rates at the organismal scale. Is coral calcification impaired by OA?</td><td>25</td></tr><tr><td>3.3. Effect on the reef net community calcification. Will coral reefs dissolve by future OA levels?</td><td>28</td></tr><tr><td>3.4. Putting my contribution into the OA debate</td><td>31</td></tr><tr><td>3.5. Conclusions and perspectives</td><td>37</td></tr><tr><td>3.6. References</td><td>38</td></tr><tr><td>4. Previous research</td><td>43</td></tr><tr><td>4.1. Response of the temperate coral Cladocora caespitosa to mid- and long-term exposure to pCO2 and temperature levels projected for the year 2100</td><td>43</td></tr><tr><td>4.2. Using volcanic CO2 seeps to study the effect of oa in a natural setting</td><td>46</td></tr><tr><td>4.2.1. The CO2 seeps site of ischia island (Gulf of Naples, Italy)</td><td>46</td></tr><tr><td>4.2.2. The CO2 seeps site of vulcano island (Sicily, Italy)</td><td>48</td></tr><tr><td>4.3. Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification</td><td>49</td></tr><tr><td>4.4. Effects of ocean acidification and high temperatures on the bryozoan Myriapora truncata at natural CO2 vents</td><td>51</td></tr><tr><td>4.5. Coral and mollusc resistance to ocean acidification adversely affected by warming</td><td>54</td></tr><tr><td>4.6. Calcification is not the achilles' heel of cold-water corals in an acidifying ocean</td><td>58</td></tr><tr><td>4.7. Physiological advantages of dwarfing in surviving extinctions in high CO2 ocean</td><td>61</td></tr><tr><td>4.8. Papua New Guinea CO2 seeps, Normanby and Ambitle islands</td><td>64</td></tr><tr><td>4.8.1. CO2 seep of Normanby Island (Upa-Upasina reef)</td><td>65</td></tr><tr><td>4.8.2. CO2 seep of Ambitle Island (Tutum Bay)</td><td>67</td></tr><tr><td>4.9. pH variability at volcanic CO2 seeps regulates coral calcifying fluid chemistry</td><td>71</td></tr><tr><td>4.10. The semi-enclosed lagoon of Bouraké</td><td>76</td></tr><tr><td>4.11. High coral recruitment despite coralline algal loss under extreme environmental conditions</td><td>81</td></tr><tr><td>4.12. Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities</td><td>84</td></tr><tr><td>5. Future research "Hope for coral reefs. Can acclimatization and adaptation follow the pace of climate change?"</td><td>87</td></tr><tr><td>5.1. Abstract</td><td>87</td></tr><tr><td>5.2. State-of-the-art and objectives</td><td>87</td></tr><tr><td>5.3. Distinguishing acclimatization from adaptation.</td><td>88</td></tr><tr><td>5.4. Objectives, originality and innovative nature of the project.</td><td>89</td></tr><tr><td>5.5. Natural analogues to study the effects of climate change.</td><td>90</td></tr><tr><td>5.6. Preliminary experiments demonstrating the value of the Bouraké system as a natural analogue</td><td>92</td></tr><tr><td>5.7. Scientific and technical programs, project organization</td><td>93</td></tr><tr><td>5.8. Experimental designs</td><td>94</td></tr><tr><td>5.9. Feasibility and mitigation strategies</td><td>101</td></tr><tr><td>5.10. Expected results of the project: high-risk/high-gain research</td><td>101</td></tr><tr><td>5.11. Size and nature of the research team and host institute</td><td>102</td></tr><tr><td>5.12. References</td><td>103</td></tr><tr><td>6. Traduction en française du paragraphe 2.3 au paragraphe 3.5</td><td>109</td></tr></table>
# 1. CURRICULUM VITAE

Riccardo Rodolfo-Metalpa, born in Genoa, Italy, July 28th 1971. Italian nationality

## EDUCATION

**Oct 2004/ Oct 2007**
**PhD Oceanography**
University of Marseille-Aix II (France); Host Institution: Centre Scientifique de Monaco (Monaco). Governmental research fellow: "Responses of two Mediterranean corals Cladocora caespitosa and Oculina patagonica to environmental and climate change". Supervisors: Prof. D. Allemand and Dr. C. Ferrier-Pagès.

**July 1993/ July 1998**
**Degree in Biological Sciences (B.Sc and M.Sc.)**
University of Genoa (Italy); one year M.Sc. at ENEA, Marine Environmental Research Centre (La Spezia, Italy): "Growth and other aspects of the biology of the coral Cladocora caespitosa (L.) (Cnidaria, Scleractinia): in situ and aquarium observations".

## EMPLOYMENT HISTORY

<table><tr><td>Oct 2013/ permanent</td><td>IRD - Institut de Recherche pour le Développement (New Caledonia). Research Scientist (CR1).</td></tr><tr><td>Mar 2013/ Sept 2013</td><td>IRD - Institut de Recherche pour le Développement (New Caledonia). Research fellow funded by LABEX Corail: "Changement climatique et production carbonée des algues Corallinacées des récifs coralliens".</td></tr><tr><td>Feb 2011/ June 2012</td><td>Marine Biology & Ecology Research Centre, University of Plymouth. Research fellow funded by EU FP7 project MedSeA grant no. 265103: "Mediterranean Sea Acidification under a changing climate".</td></tr><tr><td>Feb 2009/ May 2010</td><td>United Nations, International Agency Atomic Energy (AIEA), Monaco. Research Scientist funded by Foundation Prince Albert II of Monaco: "Effect of Ocean Acidification on Mediterranean Biodiversity". This is a competitive, ad hominem fellowship (PI position), and is not equivalent to a postdoctoral position in America or Europe.</td></tr><tr><td>Oct 2007/ Dec 2008</td><td>Marine Biology & Ecology Research Centre, University of Plymouth. Research fellow funded by Leverhulme Trust (PI JM Hall-Spencer): "Volcanic CO2 vents and Ocean Acidification".</td></tr><tr><td>Oct 2004/ Oct 2007</td><td>Centre Scientifique de Monaco (Monaco). Research fellow (PhD) funded by the host Institute: "Responses of two Mediterranean corals Cladocora caespitosa and Oculina patagonica to environmental and climate change".</td></tr><tr><td>Apr 2004/ Jul 2004</td><td>Centre Scientifique de Monaco (Monaco). Research associate "Biology and physiology of Med corals".</td></tr><tr><td>Nov 2003/ May 2004</td><td>Centre Scientifique de Monaco (Monaco). Research fellow funded by the EU programme Leonardo da Vinci I: "Biology and physiology of Mediterranean corals".</td></tr><tr><td>Feb 2002/ Feb 2003</td><td>Institute of Marine Biology and Ecology (Piombino, Italy). Research fellow funded by the EU project INTEREG III: "New methodologies for recovery and reutilisation of Posidonia oceanica (L.) drifted ashore and the monitoring of meadow".</td></tr><tr><td>Mars 2000/ Feb 2002</td><td>Marine Environmental Research Centre ENEA (La Spezia, Italy). Governmental fellowship, ENEA and CNR-MURST: "National plan for the protection of the coast and sea".</td></tr></table>
GRANTS & FELLOWS

2024 - Fonds Pacifique: *Partnering for success; Understanding coral symbiosis in the extreme reefs of New Caledonia, 64K€ (co-PI)*

2023 - Agence Nationale de la Recherche (ANR): *Nutrition as a BOOST for corals in the face of marine heat waves (BOOST) 640K€ (WP leader)*

- Fonds Pacifique: *Soaking it up: understanding climate resilience and acclimation in New Caledonian sponges, 43K€ (co-PI)*

2022 - Labex Corail: *Résilience et connectivité des coraux dans les environnements marginaux et extrêmes (RECONNECTION), 82K€ off 300 K€ (PI)*

- Fonds Pacifique: *Adaptation of calcifying algae to climate change, 32K€ (co-PI)*

2021 - Fonds Pacifique: *Adaptation of corals to climate change, 37K€ (co-PI)*

2020 - Oceanographic Cruise R/V Alis SUPERNATURAL 2: *A unique natural laboratory to study corals adaptation to global change, 200K€ (PI)*

- European Research Council (ERC) Cog Grant finalist, Travel grant to Brussels, 7K€

2019 - Oceanographic Cruise R/V Alis CARiOCA 4^th^ trip: *Acclimatation des coraux à l'acidification des océans autour de résurgences sous-marines de CO₂, 230K€ (PI)*

- European Research Council (ERC) Cog Grant finalist, Travel grant to Brussels, 7K€

- Fonds Pacifique: *Les coraux vivant dans la mangrove en Nouvelle-Calédonie auraient pu s'adapter à l'acidification et au réchauffement des océans? (SUPERCORAUX), 18K€ (PI)*

- Labex Corail: PhD fellowship project SUPERNATURAL, 44K€ (PhD Director)

- University of New Caledonia PhD fellowship project REEF-ENGINE, 44K€ (Supervisor)

- University of Bretagne-Loire PhD fellowship project ACCLICOST, 44K€ (Supervisor)

- Labex Corail: *Would super corals only be well-fed corals? (SURF), 20K€ (co-PI)*

- Breguet sponsored Oceanographic Cruise on Race for Water Hydrogen vessel: *Accumulation des MIcroplastiques dans les Coraux (AMICi) (co-PI)*

2018 - Oceanographic Cruise R/V Alis CARiOCA 3^rd^ trip: *Acclimatation des coraux à l'acidification des océans autour de résurgences sous-marines de CO₂, 230K€ (PI)*

2017 - Agence Nationale de la Recherche: *Using volcanic CO₂ vents to assess coral reefs ability to acclimatize and adapt to ocean acidification and warming (PNG-Vents), 180K€ (PI)*

- Fonds Pacifique: *Identifying the value of New Caledonia's “extreme” corals to manage reefs under climate change (COLIMATIC), 20K€ (co-PI).*

- Oceanographic Cruise R/V Alis CARiOCA 2^nd^ trip: *Acclimatation des coraux à l'acidification des océans autour de résurgences sous-marines de CO₂, 200K€ (PI)*

- Oceanographic Cruise R/V Alis Supernatural 1: *A unique natural laboratory to study corals adaptation to global change, 96K€ (PI)*

2016 - Fonds Pacifique: *Coral reef adaptation to ocean acidification (AMBITLE), 20K€ (PI).*

- Agence Nationale de la Recherche: *Coral reef acclimatization to ocean acidification at CO₂ seeps (CARiOCA), 354K€ (PI)*

- European Research Council (ERC) Cog Grant finalist, Travel grant to Brussels, 7K€

- Labex Corail: *Nouvelles priorités pour la recherche dans le domaine des changements climatiques sur les coraux constructeurs de récifs (ENSEMBLE), 12K€ (PI)*

- Oceanographic Cruise R/V Alis CARiOCA: *Acclimatation des coraux à l'acidification des océans autour de résurgences sous-marines de CO₂, 280K€ (PI)*
- Labex Corail: *Acclimatation développementale et transgénérationnelle à l'acidification des océans et réchauffement climatique (ACCLIMACID)*, **30K€** (collaborator)

- Grand Observatoire du Pacifique du Sud (GOPS): *South Ocean acidification & projected changes of coral reef ecosystems (ORACLE)*, **25K€** (PI)

2015 - Labex Corail: *Balance Autotrophie/Hétérotrophie chez les coraux (COCONUT)*, **25K€** (co-PI)

- Grand Observatoire du Pacifique du Sud (GOPS): *Coastal seawater pH oscillations: revealing the noise of coral reef communities (HONOR)*, **11K€** (PI)

2014 - European Research Council (ERC) Starting Grant finalist, Travel grant to Brussels, **7K€**

2012 - European Research Council (ERC) Starting Grant finalist, Travel grant to Brussels, **7K€**
- 3rd International Symposium on the Ocean in a High-CO2 World, Travel grant to Monterey, (USA): **1.2k€)**

2011 - Labex Corail: *Conséquences du changement climatique et des apports anthropiques sur les capacités de calcification de deux récifs coralliens (COMETA)*, **29K€** (co-PI)

2010 - Post-doc grant from the EU FP7 project MedSeA, 34K€ (PI: J Hall-Spencer)

2009 - Prince Albert II Foundation (Monaco): *Effect of Ocean Acidification on Mediterranean Biodiversity*, **40K€** (PI)

- Society for Experimental Biology Annual Meeting, Travel grant to Glasgow (UK), **0.6K€**

- 1st Mediterranean Symposium on Coralligenous conservation and other calcareous bio-concretions, Travel grant to Tabarka (Tunis), **0.4K€**

2008 - Journal of Experimental Biology, Travel grant to Eilat (Israel), **1.2K€**

- Percy Sladen Memorial Fund, Travel grant to Alicante (Spain), **0.6K€**

2004 - Centre Scientifique de Monaco (CSM), PhD fellowship, **50K€**

2003 - European award Leonardo da Vinci I for training at CSM (Monaco): **2.4K€**

## FIELD EXPERIENCES, OCEANOGRAPHIC MISSIONS

I have participated to several oceanographic cruises (SALA 3, MASSFLUX, ARCO, MEDCOR WAGNER-II, PRISTINE, AMICI), being head mission for some others (CARIOCA 1-3, AMBITLE 1-4, SUPERNATURAL 1-2); organised field missions in French Guadeloupe, French Polynesia, Red Sea, Spain, Palau and around 40 missions to study the effect of acidification using CO2 vents in the Mediterranean Sea (Ischia and Vulcano islands), in the Gulf of California, and at the mangrove extreme system in New Caledonia.

Diving professional certificate (CAH: Classe 1, mention B, and Chef de plongée scientifique, CNRS).

## INVITED SPEAKER & SEMINARIES

**32** European Research Council ERC. Interview projet ERC CoG "Hope - Hope for coral reefs. Can acclimatization and adaptation follow the pace of climate change?" 22th Sept 2020, Bruxel.

**31** European Research Council ERC. Interview projet ERC CoG "Hope - Hope for coral reefs. Can acclimatization and adaptation follow the pace of climate change?" 09th Oct 2019, Bruxel.

**30** University of Papua New Guinea UPNG. Coral reef acclimatization to ocean acidification using CO2 seeps of PNG". What can natural analogues tell us about the fate of reefs in the face of climate change? Port Moresby, PNG, 30th August 2019

**29** CEN, Conservatoire d'espaces Naturels, "Pourquoi il faut valoriser les « super coraux » de Bouraké". New Caledonia, Kone, 23 February 2018.
28 Workshop organizer. COLIMATIC - Coral associated with extreme conditions: the Bouraké case. (Fonds Pacifique), 30 participantes, Nouméa, 15-16 November 2017.

27 Fourth International Workshop on the Socio-Economic Impacts of Ocean Acidification. French Pacific Islands (Leader). 15-17 October 2017, Monaco.

26 Human impacts on Mediterranean marine ecosystems and the economy. 18-19 October 2017, Monaco.

25 French-Australian Workshop on the protection of coral reefs. Sydney 22 August 2017, Australian Maritime Museum. Assessing impacts of ocean acidification on coral reefs: 15 y of uncertainties.

24 Pacific voices for a global Ocean challenge. Session 3 – Marine and coastal ecosystems: resilience and reduction of climate change impacts. Japan-Pacific ICT Centre – USP, Suva (Fiji) 8-9 June 2017. *Assessing impacts of ocean acidification on coral reefs: 15 y of uncertainties.*

23 Workshop funded by GOPS "Vulnerability and adaptive capacity of the French Polynesian reef lagoons and society in a changing world" 18-23 April 2017, Tahiti.

22 European Research Council ERC. Interview projet ERC CoG "SELECTION - Using volcanic CO2 vents to assess marine keystone species ability to acclimatize and adapt to ocean acidification" 06th Sept 2016, Bruxel.

21 Our Common future under climate change, Paris 7-10 July, 2015 Parallel Session 1111. *Why coral reefs should care about ocean acidification: general consensus, misconceptions and future research priorities.*

20 Pacific Islands regional ocean acidification workshop, Secretariat of the Pacific Regional Environment Programme (SPREP), Auckland 7-9 October, 2015. *Why coral reefs should care about ocean acidification: general consensus, misconceptions and future research priorities.*

19 Coral reefs face global change (LaBEx CORAIL meeting), Paris 7-9 September, 2015. *Changement climatique et production carbonée des algues corallinacées des récifs coralliens.*

18 Observation systems of climate change in the South Pacific workshop (GOPS & PACE-NET+), Nouméa 11-12 June, 2015

17 European Research Council ERC. Interview projet ERC CoG "Reef Change" – Coral reef response to climate change" 12th Sept 2014, Bruxel.

16 University of Nice Sophie Antipolis (ECOMERS), Nice (France) 22th Mai, 2013. *Résistance des invertébrés a l'acidification des océans.*

15 Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE), Aix-en-Provence (France) 25th Mars, 2013. *Resilience des coraux, bryzoaires et gastéropodes a l'acidification océanique: expériences en laboratoire et près des sources volcaniques de CO2 en Méditerranée.*

14 European Research Council ERC. Interview projet ERC CoG "Reef Change" – Coral reef response to climate change" 08th May 2012, Bruxel.

13 3rd International Symposium on the Ocean in a High-CO2 world, Monterey (USA) 24-27 September, 2012. *Some Mediterranean corals, but also bryozoans, molluscs and gastropods keep calcifying at low carbonate ions concentrations.*

12 European Geophysical Union (EGU) Alexander von Humboldt International Conference; Penang (Malaysia), 20-24 June, 2011. *Corals, bryozoans and gastropods are able to calcify in acidified seawater.*

11 Station Marine d'Endoume, Centre d'Océanologie de Marseille (France) 13th February, 2012. *Coraux, bryozoaires et gastéropodes sont capables de calcifier dans des eaux plus acides.*
10 MedSea annual meeting, Vulcano (Italy) 24-27 May, 2011. *Physiological manipulations at CO₂ vents Vulcano*.

9 Universidad Nacional Autónoma de México (UNAM), Mexico City, 01-07 May, 2011. *Seawater carbonate chemistry of the Wagner and Consag basins (Bay of California), and its suitability for ocean acidification studies*.

8 Marine Biology Station of Piran (Slovenia), 24th November, 2011. *Corals, bryozoans and gastropods are able to calcify in acidified seawater*.

7 International Atomic Energy Agency (IAEA), Monaco 2nd February, 2010. *Effects of Ocean Acidification on Mediterranean shallow and deep-sea corals: Preliminary results and perspectives*.

6 The Society for Experimental Biology (SEB), Glasgow (UK) 1st July, 2009. *Physiological responses of Mediterranean corals to temperature and pH perturbations*.

5 France Embassy in Israel, Tel-Aviv, Wohl Center, Bar Ilan University (Israel) 8th December, 2009. *L'Europe, Israël et l'Union pour la Méditerranée: Les dangers de l'acidification des océans: un projet de recherche scientifique commun*.

4 Aquatic Sciences Meeting (ASLO), Nice (France) 25-30 January 2009. *Response of the Mediterranean coral Cladocora caespitosa to mid- and long-term exposure to elevated pCO₂ and temperature*.

3 International Atomic Energy Agency (IAEA), Monaco 24th Mars, 2009. *The future of scleractinian corals in a warming Mediterranean Sea*.

2 The Mediterranean Science Commission (CIESM), Menton (France) 1-4 October, 2008. *Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification*.

1 Centro Ricerche Ambiente Marino S. Teresa ENEA, La Spezia (Italy) 03rd July, 2008. *Seasonality, Climate change and ocean acidification on Mediterranean corals*.

## STUDENTS and POST DOC SUPERVISION

I have supervised 12 students (mostly during 6-month stages, M.Sc.), 2 post doc and 3 PhD.

2021 Giulia Zini (M.Sc University of Bologna); 6 months research in New Caledonia. *Role of the diffusion boundary layer in the coral metabolic regulations under extreme conditions*.

2020 Cinzia Alessi Maggioni (PhD, University New Caledonia). *Les récifs coralliens associés aux milieux extrêmes ont-ils acquis la capacité de faire face au changement global ?*

2019 Clement Tanvet (PhD, University Brest); *Acclimatation des coraux aux changements globaux : utilisation des signatures isotopiques pour évaluer l'impact de l'acidification des océans sur la calcification*

Federica Maggioni (PhD, University New Caledonia); *Dans quelle mesure les éponges peuvent-elles moduler le réseau alimentaire des écosystèmes récifaux dans un monde en évolution ?*

Rafael Valente (B.Sc. University of New Caledonia); 2 months training in New Caledonia. *Comparing classic and 3D techniques to measure coral colonies surface*.

2018 Federica Maggioni (M.Sc University of Ancona); 6 months research in New Caledonia. *Phisiological responses of sponges living at extreme conditions*.

2017 Thomas Barros (M.Sc University of Montpellier); 6 months research in New Caledonia. *Using CO₂ vents to study adaptation of corals to ocean acidification*.

Rian Prasetia (Post doc Project ANR PNG Vents); 12 months research in New Caledonia.

Julie Ripoll (Post doc Project ANR CARiOCA); 16 months research in New Caledonia.

2016 Marco Zampighi (M.Sc University of Ancona); 6 months research in New Caledonia. *Phisiological responses of corals living at extreme conditions*.
2015 Marco Zampighi (M.Sc University of Ancona); 6 months research in New Caledonia.
*Phisiological responses of corallinae algae to environmental stress.*

Virginia Nguyen (B.Sc University of New Caledonia); 3 months research in New Caledonia. *Calcification rates of corals and crustose corallinae algae.*

**Amaury Durbano** (B.Sc University of New Caledonia); 3 months research in New Caledonia. *Calcification rates of corals and crustose corallinae algae.*

2014 **Lara Cavalié** (B.Sc University of New Caledonia); 3 months research in New Caledonia.
*Effects of ocean acidification on corals and crustose corallinae algae.*

2013 **Julien Debreuil** (Post Doc); 12 months research in New Caledonia. *Effects of ocean acidification on corals and crustose corallinae algae.*

Tom Biscéré (M.Sc Université de Bretagne Occidentale) 6 months research in New Caledonia. *Effets de l'acidification des océans et de concentrations plus élevées en cobalt sur la croissance et l'efficacité photosynthétique de deux espèces de coraux.*

<2013 **Four M.Sc:** one in Monaco (Adrien Delval, University of Paris VI), two in UK (CR Warner-Holder and Cecilia Baggini, University of Plymouth), one in Italy (Roberto Sicari, University of Palermo).

## RADIO and TV COMMUNICATIONS

YouTube repository: https://www.youtube.com/channel/UCCOPiCJpW6VVEFSr_mbHoLA

Twitter account: https://twitter.com/RMetalpa

Project CARIOCA (IRD): https://www.youtube.com/watch?v=tfoT6TZfbNw&t=143s

Project CARIOCA (P-Y Cousteau): https://www.youtube.com/watch?v=TcNpQvc0cPw

Project ACCLICOST (BRUToriginal): https://www.youtube.com/watch?v=Psdf2npwYkQ

Project AMICi, Race for Water: https://www.youtube.com/watch?v=EXzyIdIVOfc

Project SUPERNATURAL (IRD): https://www.youtube.com/watch?v=eKbqekbpdps

Project COCONUT (IRD): https://www.youtube.com/watch?v=fpKzaM2JXcE

Journal 20h NC1ere: https://www.youtube.com/watch?v=xLCLLS6ImQI

Journal 20h NC1ere: https://www.youtube.com/watch?v=dW5Og5lG_Yo

Journal 20h NC1ere: https://www.youtube.com/watch?v=jGEP_0nDRLc

Journal 20h NC1ere: https://www.youtube.com/watch?v=nTWhyFs1z_o

Radio NC1ere: https://www.youtube.com/watch?v=kWRe97wUQHo

TV NCTV: https://www.youtube.com/watch?app=desktop&v=DOG-qsvHRPg&t=934s

TV France 2 «13h15 le dimanche» https://www.francetvinfo.fr/replay-magazine/france-2/13h15/13h15-du-dimanche-25-octobre-2020_4131597.html

TV Science et Vie : https://www.facebook.com/ScienceetvieTV/videos/lhomme-au-chevet-des-coraux-extrait-4/636931013495328/

Canal Plus « Un Paradis en péril » : https://www.youtube.com/watch?v=OvdKihHGt44

## PUBLICATIONS

71 peer-reviewed publications. Number of citations 6431 (Google scholar), index h=39

2023

71 Maggioni F, Bell JJ, Pujo-Pay M, Shaffer M, Cerrano C, Lemonnier H, Letourneur Y, Rodolfo-Metalpa R. Sponge organic matter recycling: Reduced detritus production under extreme environmental conditions. **Marine Pollution Bulletin** 190:114869.

70 Tanvet C, Camp EF, Sutton J, Houlbrèque F, Thouzeau G, Rodolfo-Metalpa R. Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities. **Ecology and Evolution** 13(5): e10099.

69 Schoepf V, Baumann JH, Barshis DJ, Browne NK, Camp EF, Comeau S, Cornwall CE, Guzmán HM, Riegl B, Rodolfo-Metalpa R, Sommer B. Corals at the edge of
environmental limits: A new conceptual framework to re-define marginal and extreme coral communities. **Science of the Total Environment** 163688.

68 Reimer JD, Agostini S, Golbuu Y, Harvey BP, Izumiyama M, Jamodiong EA, Kawai E, Kayanne H, Kurihara H, Ravasi T, Wada S, Rodolfo-Metalpa R. High abundances of zooxanthellate zoantharians (Palythoa and Zoanthus) at multiple natural analogues: potential model anthozoans? **Coral Reefs** 15: 1-9.

2022

67 Bell JJ, Shaffer M, Luter HM, Mana R, Rodolfo-Metalpa R. Phototrophic sponge productivity may not be enhanced in a high CO₂ world. **Global Change Biology** 28(16): 4900-4911.

66 Tanvet C, Benzoni F, Peignon C, Thouzeau G, Rodolfo-Metalpa R. High coral recruitment despite coralline algal loss under extreme environmental conditions. **Frontiers in Marine Science** 9.

65 Jaquemont J, Houlbrèque F, Tanvet C, Rodolfo-Metalpa R. Long-term exposure to an extreme environment induces specie-specific responses in corals' photosynthesis and respiration rates. **Marine Biology** 169(6): 1-5.

64 Meunier V, Bonnet S, Camps M, Benavides M, Dubosc J, Rodolfo-Metalpa R, Houlbrèque F. Ingestion of diazotrophs makes corals more resistant to heat stress. **Biomolecules** 12: 537.

63 Kang J, Nagelkerken I, Rummer JL, Rodolfo-Metalpa R, Munday PL, Ravasi T, Schunter C. Rapid evolution fuels transcriptional plasticity to ocean acidification. **Global Change Biology** 28(9): 3007-3022.

62 Comeau S, Cornwall CE, Shlesinger T, Hoogenboom M, Mana R, McCulloch MT, Rodolfo-Metalpa R. pH variability at volcanic CO₂ seeps regulates coral calcifying fluid chemistry. **Global Change Biology** 28(8): 2751-2763.

2021

61 Geissler L, Meunier V, Rädecker N, Perna G, Rodolfo-Metalpa R, Houlbrèque F, Voolstra, CR. Highly variable and non-complex diazotroph communities in corals from ambient and high CO₂ environments. **Frontiers in Marine Science** 8: 754652.

60 Ponti M, Linares C, Cerrano C, Rodolfo Metalpa R, Hoeksema BW. Biogenic reefs at risk: Facing globally widespread local threats and their interaction with climate change. **Frontiers in Marine Science** 8: 793038.

59 Agostini S, Houlbrèque F, Biscéré T, Harvey BP, Heitzman JM, Takimoto R, Yamazaki W, Milazzo M, Rodolfo-Metalpa R. Greater Mitochondrial Energy Production Provides Resistance to Ocean Acidification in “Winning” Hermatypic Corals. **Frontiers in Marine Science** 7: 600836.

58 Maggioni F, Pujo-Pay M, Aucan J, Cerrano C, Calcinai B, Payri C, Benzoni F, Letourneur Y, Rodolfo-Metalpa R. The Bouraké semi-enclosed lagoon (New Caledonia), a natural laboratory to study the lifelong adaptation of a coral reef ecosystem to extreme environmental conditions. **Biogeosciences** 18(18): 5117-5140.

57 Meunier V, Geissler L, Bonnet S, Rädecker N, Perna G, Grosso O, Lambert C, Rodolfo-Metalpa R, Voolstra CR, Houlbrèque F. Microbes support enhanced nitrogen requirements of coral holobionts in a high CO₂ environment. **Molecular Ecology** 30(22): 5888-5899.

2020

56 Horwitz R, Norin T, Watson SA, Pistevos JC, Beldade R, Hacquart S, Gattuso JP, Rodolfo-Metalpa R, Vidal-Dupiol J, Killen SS, Mills SC. Near-future ocean warming and acidification alter foraging behaviour, locomotion, and metabolic rate in a keystone marine mollusc. **Scientific Reports** 10(1): 5461.
55 Bang IC, Bruggemann F, Chen C. et al. Microsatellite records for volume 12, issue 2. **Conservation Genet Resour** 12, 337–351. https://doi.org/10.1007/s12686-020-01148-6

54 Gaubert J, Rodolfo-Metalpa R, Greff S, Thomas OP, Payri CE. Impact of ocean acidification on the metabolome of the brown macroalgae *Lobophora rosacea* from New Caledonia. **Algal Research** 46: 101783.

53 Camp EF, Suggett DJ, Pogoreutz C, Nitschke MR, Houlbreque F, Hume BC, Gardner SG, Zampighi M, Rodolfo-Metalpa R, Voolstra CR. Corals exhibit distinct patterns of microbial reorganisation to thrive in an extreme inshore environment. **Coral Reefs** 3: 1-6.

2019

52 Bambridge T, Chlous F, D'arcy P, Claudet J, Pascal N, Reynaud S, Rodolfo-Metalpa R, Tambutté S, Thomassin A, Virto LR. Society-based solutions to coral reef threats in French pacific territories. **Regional Studies in Marine Science** 29:100667.

51 Biscéré T, Zampighi M, Lorrain A, Jurriaans S, Foggo A, Houlbrèque F. Rodolfo-Metalpa R. High p CO2 promotes coral primary production. **Biology letters** 15(7): 20180777.

50 DeCarlo TM, Harrison HB, Gajdzik L, Alaguarda D, Rodolfo-Metalpa R, D'Olivo J, Liu G, Patalwala D. McCulloch MT. Acclimatization of massive reef-building corals to consecutive heatwaves. **Proceedings of the Royal Society B** 286 (1898): 20190235.

49 Pichler T, Biscéré T, Kinch J, Zampighi M, Houlbrèque F, Rodolfo-Metalpa R. Suitability of the shallow water hydrothermal system at Ambitle Island (Papua New Guinea) to study the effect of high pCO2 on coral reefs. **Marine Pollution Bulletin** 138: 148-158.

2018

48 Langer G, Sadekov A, Nehrke G, Baggini C, Rodolfo-Metalpa R, Hall-Spencer JM, Cuoco E, Bijma J, Elderfield H. Relationship between mineralogy and minor element partitioning in limpets from an Ischia CO2 vent site provides new insights into their biomineralization pathway. **Geochimica et Cosmochimica Acta** 236: 218-229.

47 Camp EF, Schoepf V, Mumby PJ, Hardtke LA, Rodolfo-Metalpa R, Smith DJ, Suggett DJ. The future of coral reefs subject to rapid climate change: lessons from natural extreme environments. **Frontiers in Marine Sciences** 5(4). doi: 10.3389/fmars.2018.00004.

46 Tribollet A, Grange JS, Parra H, Rodolfo-Metalpa R, Carreiro-Silva M. Limited carbonate dissolution by boring microflora at two volcanically acidified temperate sites: Ischia (Italy, Mediterranean Sea) and Faial (Azores, NE Atlantic Ocean). **Global Biogeochemical Cycles**, 32(1): 78-91.

2017

45 Biscéré T, Lorrain A, Rodolfo-Metalpa R, Gilbert A, Wright A, Devissi C, Peignon C, Farman R, Duvieilbourg E, Payri C, Houlbrèque F. Nickel and ocean warming affect scleractinian coral growth. **Marine Pollution Bulletin** 120 (1-2): 250-258.

44 Courtial L, Ferrier- Pages C, Jacquet S, Rodolfo-Metalpa R, Reynaud S, Rottier C, Houlbreque F. Temperature and UVR effects on the organic matter fluxes and the metabolic activity of *Acropora muricata*. **Biology Open** 6: 1190-1199.

43 Camp EF, Nitschke MR, Rodolfo-Metalpa R, Houlbreque F, Gardner SG, Smith SJ, Zampighi M, Suggett DJ. Reef-building corals thrive within hot-acidified and deoxygenated waters. **Scientific Reports** 7: 2434 | DOI:10.1038/s41598-017-02383-y.

2016

42 Ventura P, Jarrold MD, Merle PL, Barnay-Verdier S, Zamoum T, Rodolfo-Metalpa R, Calosi P, Furla P. Changes in dissolved inorganic carbon preference confer sea anemones resilience to ocean acidification. **Marine Ecology Progress Series** 559:257-263.
41 Milazzo M, Cattano C, Alonzo SH, Foggo A, Cristina M, Rodolfo-Metalpa R, Sinopoli M, Spatafora D, Stiver KA, Hall-Spencer JM. Ocean acidification affects fish spawning but not paternity at CO2 seeps. **Proceeding Royal Society B** doi: 10.1098/rspb.2016.1021.

40 Fruchter N, Eisenhauer A, Dietzel M, Fietzke J, Böhm F, Montagna P, Stein M, Lazar B, Rodolfo-Metalpa R, Erez J. ⁸⁸Sr/⁸⁶Sr fractionation in inorganic aragonite and in corals. **Geochimica et Cosmochimica Acta** 178: 268-280.

2015

39 Biscéré T, Rodolfo-Metalpa R, Lorrain A, Chauvaud L, Thébault J, Clavier J, Houlbrèque F. Responses of two scleractinian corals to cobalt pollution and ocean acidification. **PLoS ONE** doi:10.1371/journal.pone.0122898

38 Garilli V, Rodolfo-Metalpa R, Scuderi D, Brusca L, Parrinello D, Rastrick SPS, Foggo A, Twitchett RJ, Hall-Spencer JM, Milazzo M. Physiological advantages of dwarfing in surviving extinctions in high CO₂ oceans. **Nature Climate Change** doi:10.1038/NCLIMATE2616

37 Rodolfo-Metalpa R, Montagna P, Aliani S, Borghini M, Canese S, Hall-Spencer JM, Foggo A, Milazzo M, Taviani M, Houlbreque F. Calcification is not the Achilles' heel of cold-water corals in an acidifying ocean. **Global Change Biology** 21: 2238-2248. doi:10.1111/gcb.12867

36 Houlbrèque F, Reynaud S, Godinot C, Oberhänsli F, Rodolfo-Metalpa R, Ferrier-Pagès C. Ocean acidification reduces feeding rates in the scleractinian coral *Stylophora pistillata*. **Limnology & Oceanography 60**(1) doi:10.1002/lno.10003

35 Chauhan A, Pathak A, Rodolfo-Metalpa R, Milazzo M, Green SJ, Hall-Spencer JM. 2014. Metagenomics reveals planktonic bacterial community shifts across a natural CO2 gradient in the Mediterranean Sea. **Genome Announcement** 3(1):e01543-14. doi:10.1128/genomeA.01543-14

2014

34 Rodolfo-Metalpa R, Hoogenboom MO, Rottier C, Ramos-Esplá A, Baker AC, Fine F, Ferrier-Pagès C. Thermally tolerant corals have limited capacity to acclimatize to future warming. **Global Change Biology** 20(10): 3036-3049

33 Milazzo M, Rodolfo-Metalpa R, Bin San Chan V, Fine M, Alessi C, Thiyagarajan V, Hall-Spencer JM, Chemello R. Ocean acidification impairs vermetid reef recruitment. **Scientific Reports** 4, 4189; DOI: 10.1038/srep04189

32 Montagna P, McCulloch M, Douville E, Lopez Correa M, Trotter J, Rodolfo-Metalpa R, Dissard D, Ferrier-Pageès C, Frank N, Freiwald A, Goldstein S, Mazzoli C, Reynaud S, Ruggeberg A, Russo S, Taviani M. Li/Mg systematics in scleractinian corals: Calibration of the Thermometer. **Geochimica et Cosmochimica Acta** http://dx.doi.org/10.1016/j.gca.2014.02.005

31 Ziveri P, Passaro M, Incarbona M, Milazzo, Rodolfo-Metalpa R, Hall-Spencer JM. Decline in coccolithophore diversity and impact on coccolith morphogenesis along a natural CO₂ gradient. **Biological Bulletin** 226: 282-290

30 Langer G, Nehrke G, Baggini C, Rodolfo-Metalpa R, Hall-Spencer JM., Bijma J. Limpets counteract ocean acidification induced shell corrosion by thickening of aragonitic shell layers. **Biogeosciences** 11: 7363-7368

2013

29 Goodwin C, Rodolfo-Metalpa R, Picton B, Hall-Spencer JM. Effects of ocean acidification on sponge communities. **Marine Ecology** 32 doi: 10.1111/maec.12093

28 Prol-Ledesma RM, Torres-Vera M-A, Rodolfo-Metalpa R, Ángeles C, Lechuga Deveze CH, Villanueva-Estrada RE, Shumilin E, Robinson C. High heat flow and ocean acidification in nascent spreading centers in the northern Gulf of California, Mexico. **Nature Communication** 4(1388) doi:10.1038/ncomms2390
27 Pettit LR, Hart MB, Medina-Sánchez AN, Smart CW, Rodolfo-Metalpa R, Hall-Spencer JM, Prol-Ledesma RM. Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California, Mexico. Marine Pollution Bulletin DOI:10.1016/j.marpolbul.2013.02.011

26 Boatta F, D'Alessandro W, Gagliano AL, Liotta M, Milazzo M, Rodolfo-Metalpa R, Hall-Spencer JM, Parello F. Geochemical survey of Levante Bay, Vulcano Island (Italy), a natural laboratory for the study of ocean acidification. Marine Pollution Bulletin DOI: 10.1016/j.marpolbul.2013.01.029

2012

25 Hahn S, Rodolfo-Metalpa R, Griesshaber E, Schmal WW, Buhl D, Hall-Spencer JM, Baggini C, Immenhauser A. Marine bivalve geochemistry and shell ultrastructure from modern low pH environments as possible archives of past ocean acidification. *Biogeosciences* 9: 1897-1914

24 Suggett DJ, Hall-Spencer JM, Rodolfo-Metalpa R, Boatman TG, Payton R, Pettay DT, Johnson VR, Warner ME (2012). Sea anemones may thrive in a high CO₂ world. **Global Change Biology** 18(10): 3015-3025

23 Meron D, Rodolfo-Metalpa R, Cunningham R, Baker AC, Fine M, Banin E. Changes in coral microbial communities in response to a natural pH gradient. **The ISME Journal** 6(9): 1775-1785

22 Houlbrèque F, Rodolfo-Metalpa R, Jeffree R, Oberhänsli F, Teyssié JL, Boisson F, Al-Trabeen K, Ferrier-Pagès C. (2012) Effects of increased pCO₂ on zinc uptake and calcification in the tropical coral *Stylophora pistilla*. **Coral Reefs** 31 (1):101-109.

2011

21 Ferrier-Pagès C, Peirano A, Abbate M, Cocito S, Negri A, Rottier C, Riera P, Rodolfo-Metalpa R, Reynaud S. Summer autotrophy and winter heterotrophy in the temperate symbiotic coral *Cladocora caespitosa*. **Limnology & Oceanography** 56(4): 1429-1438

20 Trotter J, Montagna P, McCulloch P, Silenzi S, Reynaud S, Mortimer G, Martin S, Ferrier-Pagès C, Gattuso J-P, Rodolfo-Metalpa R. Quantifying the pH ‘vital effect’ in the temperate zooxanthellate coral *Cladocora caespitosa*: Validation of the boron seawater pH proxy. **Earth and Planetary Science Letters** 303: 163-173

19 Rodolfo-Metalpa R, Houlbrèque F, Tambutté E, Boisson F, Baggini C, Patti FP, Jeffree R, Fine M, Foggo A, Gattuso J-P, Hall-Spencer JM. Coral and mollusc resistance to ocean acidification adversely affected by warming. **Nature Climate Change** 1: 308-312

18 Lombardi C, Rodolfo-Metalpa R, Cocito S, Gambi MC, Taylor PD. Structural and geochemical alterations in the Mg calcite bryozoan *Myriapora truncata* under elevated seawater pCO₂ simulating ocean acidification. **Marine Ecology** 32(2): 211-221

2010

17 Hoogenboom M, Rodolfo-Metalpa R, Ferrier-Pagès C. Co-variation between autotrophy & heterotrophy in the Mediterranean coral *Cladocora caespitosa*. **The Journal of Experimental Biology** 213: 2399-2409

16 Rodolfo-Metalpa R, Lombardi C, Cocito S, Hall-Spencer J, and Gambi MC. Effects of ocean acidification and high temperatures on the bryozoan *Myriapora truncata* at natural CO₂ vents. **Marine Ecology** 31: 447-456

15 Cigliano M, Gambi MC, Rodolfo-Metalpa R, Patti F, Hall-Spencer J. Effects of ocean acidification on invertebrate settlement at natural volcanic CO₂ vents. **Marine Biology** 157: 2489-2502

14 Rodolfo-Metalpa R, Martin S, Ferrier-Pages C, Gattuso J-P. Response of the temperate coral *Cladocora caespitosa* to mid- and long-term exposure to pCO₂ and temperature levels projected for the year 2100. **Biogeosciences** 7: 289-300
**2009**

13 Rollion-Bard C, Vigier V, Meibom A, Blamart D, Reynaud S, Rodolfo-Metalpa R, Martin S, Gattuso J-P. Effect of environmental conditions and skeletal ultrastructure on the Li isotopic composition of scleractinian corals. **Earth and Planetary Science Letters** 286: 63-70

**2008**

12 Martin S, Rodolfo-Metalpa R, Ransome E, Rowley SJ, Buia MC, Gattuso JP, Hall-Spencer JM. Effects of naturally acidified seawater on seagrass calcareous epibionts. **Biology Letters** 4: 689-692

11 Hall-Spencer JM, Rodolfo-Metalpa R, Martin S, Ransome E, Fine M, Turner SN, Rowley SJ, Tedesco D, Buia MC Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification. **Nature** 454: 96-99

10 Rodolfo-Metalpa R, Reynaud S, Allemand D, Ferrier-Pagès C. Temporal and depth response of two temperate corals, *Cladocora caespitosa* and *Oculina patagonica* from the North Mediterranean Sea. **Marine Ecology Progress Series** 369: 103-114

9 Rodolfo-Metalpa R, Huot Y, Ferrier-Pagès C. Photosynthetic response of the Mediterranean zooxanthellate coral *Cladocora caespitosa* to different light and temperature conditions. **The Journal of Experimental Biology** 211: 1579-1586

8 Rodolfo-Metalpa R, Peirano A, Houlbrèque F, Abbate M, Ferrier-Pagès C. Effect of temperature, light and heterotrophy on the growth rate and budding of the temperate coral *Cladocora caespitosa*. **Coral Reefs** 27: 17-25

**<2006**

7 Rodolfo-Metalpa R, Richard C, Allemand D, Ferrier-Pagès C (2006). Growth and photosynthesis of two Mediterranean corals *Cladocora caespitosa* and *Oculina patagonica* under normal and elevated temperatures. **The Journal of Experimental Biology** 209: 4546-4556

6 Rodolfo-Metalpa R, Richard C, Allemand D, Bianchi CN, Morri C, Ferrier-Pagès C (2006). Response of zooxanthellae in symbiosis with the Mediterranean corals *Cladocora caespitosa* and *Oculina patagonica* to elevated temperatures. **Marine Biology** 150: 45-55

5 Peirano A, Abbate M, Cerrati G, Difesca V, Peroni C, Rodolfo-Metalpa R (2005). Monthly variations in calyx growth, polyp tissue, and density banding of the Mediterranean scleractinian *Cladocora caespitosa* (L.). **Coral Reefs** 24(3): 404-409

4 Rodolfo-Metalpa R, Bianchi CN, Peirano A, Morri C (2005). Tissue necrosis and mortality of the temperate coral *Cladocora caespitosa* (L. 1767). **Italian Journal of Zoology** 72: 271-276

3 Peirano A, Morri C, Bianchi CN, Rodolfo-Metalpa (2001). Biomass, carbonate standing stock and production of the Mediterranean coral *Cladocora caespitosa*. **Facies** 44:75-80.

2 Rodolfo-Metalpa R, Bianchi CN, Peirano A, Morri C (2000). Coral mortality in NW Mediterranean. **Coral Reefs** 19: 24

1 Rodolfo-Metalpa R, Peirano A, Morri C, Bianchi CN (1999); Coral calcification rates in the Mediterranean Scleractinian coral *Cladocora caespitosa* (L., 1767). **Advances in Oceanography and Limnology** 13(1): 291-299

**BOOK CHAPTERS**

7 Rodolfo-Metalpa R, Houlbrèque F, Payri C. (2018) Super corals in New Caledonia can resist climate change. Nouvelle-Calédonie Archipel de Corail. IRD Editions, Marseille, 155:160.
6 Biscéré T, Lorrain A, Rodolfo-Metalpa R, Farman R, Gilbert A, Wright A, Houlbrèque F. (2018) Impact of metals on coral reefs. Nouvelle-Calédonie Archipel de Corail. IRD Editions, Marseille, 151:154.

5 Houlbreque F, Rodolfo-Metalpa R, Ferrier-Pagès C. (2016) Heterotrophic nutrition of tropical, temperate and deep-sea corals. In: Downs AC, Woodley CM, Bruckner AW, Porter JW (Eds) Diseases of Corals. Chapter 10. Wiley-Blackwell 150-163.

4 Hall-Spencer JM & Rodolfo-Metalpa R (2012). Effects of ocean acidification on Mediterranean coastal habitats. In Stambler N (ed.) *Life in the Mediterranean Sea: A look at habitat changes*. Nova Science Publishers Inc. p 671-684. ISBN: 978-1-61209-644-5.

3 Hall-Spencer JM & Rodolfo-Metalpa R (2008). Loss of Mediterranean marine biodiversity in a high-CO₂ world. In: Briand F (Ed.), Impacts of acidification on biological, chemical and physical systems in the Mediterranean and Black Seas, pp. 5-30. Monaco: CIESM Workshop Monographs No. 36: 61-64.

2 Yilmaz A, De Lange G, Dupont S, Fine M, Fowler SW, Gattuso J-P, Gazeau F, Gehlen M, Goyet C, Jeffree R, Montagna P, Rees AP, Reynaud S, Rodolfo-Metalpa R, Ziveri P, Briand F (2008). Executive summary. In: Briand F (Ed.), Impacts of acidification on biological, chemical and physical systems in the Mediterranean and Black Seas, pp. 5-30. Monaco: CIESM Workshop Monographs No. 36:5-19.

1 Cocito S, Chiantore M, & Santangelo G, Bavestrello G, Cerrano C, Mistri M, Peirano A, Pannacciulli F, Rodolfo-Metalpa R, Zupo V (1999). Monitoraggio delle popolazioni animali naturali. **Biologia Marina Mediterranea** 10 (suppl.): 327-366.

PUBLICATIONS (no impact factor)

16 Sabrié ML, et al., (2019). Science et développement durable: 75 ans de recherche au Sud. https://www/documentation.ird.fr/hor/fdi:010075823

15 Meyer JY, Letourneur Y, Payri C, Taquet M, Vidal E, André L, Dumas P, Gaertner J-C, Gerbeaux P, McCoy S, Pauly D, Rodolfo-Metalpa R, Steven A, Thaman R, Van Wynsberge S (2019). Partie 1 : Biodiversité, services écosystémiques et activités socio-économiques basées sur les ressources naturelles. In : Payri C, et Vidal E (eds.) *Biodiversité en Océanie, un besoin urgent d'action, Nouméa 2019*. Nouméa : Presses universitaires de la Nouvelle-Calédonie, 17-29. Atelier Biodiversité en Océanie, Nouméa (NCL) ISBN 979-10-910321-1-7

14 Hardy S, Sabrié M, Mourier T, Lavagne C, Thivent V, Guérin I, Roubaud F, Guilmoto C, Razafindrakoto M, Pilon M, Léglise I. et al., (2019) Cartographier les ressources pour gérer les priorités. https://hal.ird.fr/ird-02272452

13 Taviani M, Angeletti L, Antolini B, Ceregato A, Evans J, Froglia C, Gandolfi L, Garilli V, Giampieri C, Hernandez Goldstein E, Leidi E, Lorenzini F, Maier C, Montagna P, Naumann M, Rodolfo-Metalpa R, Rosso A, Zammit PP, Zibrowius H. (MedCor cruise Shipboard Staff). Hermione cruise MEDCOR of RV Urania supplies new significant information on deep-sea ecosystems in the Strait of Sicily (deep-sea corals, cold seeps and human impact). **Hermione annual meeting**, 12-16 April, 2010, Attard, Malta.

12 Gambi MC, Hall-Spencer JM, Cigliano M, Cocito S, Lombardi C, Lorenti M, Patti FP, Porzio L, Rodolfo-Metalpa R, Scipione MB, Buia MC (2010). Using volcanic marine CO₂ vents to study the effects of ocean acidification on benthic biota: highlights from Castello Aragonese. **Biologia Marina Mediterranea**, 17(1):86-88.

11 Cigliano M, Rodolfo-Metalpa R, Patti FP, Hall-Spencer JM, Gambi MC (2009). "Message in the bubble": first data of water acidification effects on motile invertebrates under natural CO₂ vents conditions. **Biologia Marina Mediterranea** 16 (1): 254-255.

10 Rodolfo-Metalpa R, Peirano A, Ferrier-Pagès C (2009). The future of mediterranean scleractinian corals in a warming mediterranean sea. Proceedings of the 1st Mediterranean
symposium on the conservation of the coralligenous and other calcareous bio-
concretions, Tabarka (Tunisia) 15–16 January, 2009.

9 **Rodolfo-Metalpa R** & Hall-Spencer JM (2008). Ecological effects of volcanic CO2 vents. **JMBA Global Marine Environment** 8:21-23.

8 Peirano A, **Rodolfo-Metalpa R**, Morri C (2002). Il corallo *Cladocora caespitosa* (Anthozoa, Scleractinia) in Mediterraneo. **Biologia Marina Mediterranea** 8(1):259-263.

7 Rodolfo-Metalpa R (2002). Ricerca scientifica e acquariofilia. **Hydra** 8: 38-41.

6 Fontani S. & Rodolfo-Metalpa R (2002). Campagna Oceanografica SALE 3:11-28 agosto 2000. Volume ENEA, 88 pp.

5 Rodolfo-Metalpa R, Abbate M, Peirano A, Bianchi CN (2002). Crescita in acquario di coralli mediterranei. **Energia, Ambiente e Innovazione** 2/2002:86-87.

4 Rodolfo-Metalpa R, Abbate M, Peirano A, Bianchi CN, Cerrati G, Di Fesca V (2002). Cambiamenti climatici e ambiente marino: effetti della temperatura e della salinità sulla crescita in acquario del corallo *Cladocora caespitosa*. ENEA RT/CLIM/22:1-21.

3 Rodolfo-Metalpa R (2002). Quanto cresce la mia Acropora? **Hydra** 18: 25-27.

2 Rodolfo-Metalpa R (2001). L'influenza di Luce, Temperatura e Alimentazione sulla crescita dei coralli mediterranei *Cladocora caespitosa* e *Balanophyllia europaea*. ENEA RT/AMB/2001/02.

1 Morri C, Peirano A, Bianchi CN, Rodolfo-Metalpa R (2002). On going studies on the Mediterranean coral *Cladocora caespitosa*. Reef Encounter 27:22-25.

CONFERENCE TALKS (the ones I remember!)

30 Rodolfo-Metalpa R., Shlessinger T., Maggioni F., Benzoni F. Hoogenboom M. Revising the consensus about the reduction in coral diversity and structurally complex reefs due to ocean acidification. 5th Asia-Pacific Coral Reef Symposium. 22 June 2023, Singapour.

29 Alessi C., Lemonnier H., Camp E., Wabete N., Payri C., Rodolfo-Metalpa R. Intra-clade diversity provide resistance to bleaching under extreme environmental conditions in *Acropora muricata*. 5th Asia-Pacific Coral Reef Symposium. 20 June 2023, Singapour

28 Maggioni F, Bell J. J, Pujo-Pay M, Shaffer M, Cerrano C, Lemonnier H, Letourneur Y, Rodolfo-Metalpa R. Shift in sponge organic matter recycling and symbiont photosynthetic activity under extreme environmental conditions. 11th International sponge symposium, 10-14 October 2022, Leiden (Netherlands).

27 Tanvet C, Dissard D, Guillermic M, Sutton J, Thouzeau G, Butscher J, Dubosc J, Rodolfo-Metalpa R. First long-term (~30 years) coral acclimatization to ocean acidification recorded in geochemical signatures: A case study of New Caledonia. International Coral Reef Symposium, 03-08 July 2022, Bremen, Germany (Oral).

26 Tanvet C, Sutton J, Guillermic M, Govan S, Thouzeau G and Rodolfo-Metalpa R. Physiological plasticity of corals long-life acclimatized to contrasting pH conditions. International Coral Reef Symposium, 19-23 July 2021, Bremen, Germany.

25 Maggioni F, Bell J. J, Pujo-Pay M, Cerrano C, Letourneur Y, Rodolfo-Metalpa R. Effect of high pCO2 and low oxygen on the sponge loop in a mangrove lagoon in New Caledonia. 14th International Coral Reef Symposium (ICRS), Bremen (Germany), 21 July 2021.

24 Majorel C, Maggioni F, Jourand P, Rodolfo-Metalpa R. SAR202 clade of the phylum Chloroflexi dominates the Rhabdastrella globostellata sponge microbiome exposed to sub-optimal environmental conditions. 14th International Coral Reef Symposium (ICRS), Bremen (Germany), 19 July 2021.

23 Agostini S, Harvey B, Takimoto R, Heitzman J, Houlbreque F, Rodolfo-Metalpa R. Is mitochondrial activity a factor for resistance to ocean acidification on corals? 1st Japanese Coral Reef Symposium, Nishihara-machi (Okinawa), 24 November 2018.
22 Ripoll J, Berteaux-Lecellier V, Biscéré T, Lecellier G, Rodolfo-Metalpa R. Acclimation of three coral species to ocean acidification around natural CO2 seeps: transplantation & gene expression. APCRS, 4th Asia-Pacific Coral Reef Symposium, 4-8/06/2018.

21 Pichler T, Rodolfo-Metalpa R, & the CARIOCA team (Biscéré T, Bourgeois B, Houlbreque F, Lorrain A, Kinch J, Peignon C). Understanding coral reef acclimatization to ocean acidification at submarine CO2 seeps, Tutum Bay, Ambitle Island, Papua New Guinea. 3rd European Conference on Scientific Diving – March, 22/23 – 2017. Funchal – Madeira – Portugal.

20 Camp EF, Smith DJ, Rodolfo-Metalpa R, Suggett DJ. Mangrove habitats select coral populations tolerant of ph and temperature conditions predicted for reefs under climate change. 13th International Coral Reef Symposium (Hawaii) 19-24 June, 2016.

19 Biscere T, Rodolfo-Metalpa R, Lorrain A, Chavaud L, Thébault J, Gilbert A, Clavier J, Houlbrèque F. Responses of two scleractinian corals to metal pollution and ocean acidification. 13th International Coral Reef Symposium (Hawaii) 19-24 June, 2016.

18 Houlbrèque F, Rodolfo-Metalpa R, Jeffree R, Oberhansli F, Teyssie JL, Boisson F, Ferrier-Pages C. Zinc: a bit of a boost to resist to warming! 12th ISRS International Coral Reef Symposium. Cairns (Australia) 9-13 July, 2012.

17 Reynaud S, Peirano A, Cocito S, Rodolfo-Metalpa R, Ferrier-Pagès C. Seasonal shift from autotrophy to heterotrophy in a temperate symbiotic coral, Cladocora caespitosa. 12th International Coral Reef Symposium. Cairns (Australia) 9-13 July, 2012.

16 Hahn S, Rodolfo-Metalpa R, Griesshaber E, Schmal WW, Buhl D, Hall-Spencer JM, Baggini C, Immenhauser A. The geochemistry and shell ultrastructure of bivalves as archives of past ocean acidification events: recent Mytilidae in a natural acidic setting in Ischia (Italy, Mediterranean Sea). EGU General Assembly 2011, Vienna (Austria) 03-08 April, 2011.

15 Immenhauser A, Hahn S, Rodolfo-Metalpa R, Griesshaber E, Schmahl W, Buhl D, Hall-Spencer JM, Baggini C. Geochemistry and shell ultrastructure of bivalves: Archives of ocean acidification. 28th IAS Meeting Zaragoza (Spain) 5-8 July, 2011.

14 Houlbreque F, Rodolfo-Metalpa R, Ferrier-Pages C, Boisson F, Al-Trabeen K, Oberhansli F, Jeffree R. Effects of increased pCO₂ on zinc bioaccumulation and calcification in the tropical coral Stylophora pistillata. Ocean Sciences Meeting, Portland (USA), 22-26 February, 2010.

13 Hall-Spencer JM, Rodolfo-Metalpa R, Buia MC, Cigliano M, Cocito S, Lombardi C, Lorenti M, Martin S, Patti FP, Porzio L, Scipione MB, Gambi MC. Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification. EMBS "European Marine Biology Symposium"; Edinburgh (UK) 23-27 August, 2010.

12 Rodolfo-Metalpa R, Houlbrèque F, Tambutté E, Boisson F, Fine M, Foggo A, Gattuso J-P, Hall-Spencer JM. Calcification of two shallow-water Mediterranean corals in the face of ocean acidification. ISRS "International Society for Reef Studies" European Meeting; Wageningen (Germany) 13-17 December, 2010.

11 Hall-Spencer JM, Pettit L, Johnston V, Graziano MG, Rodolfo-Metalpa R. Volcanic CO₂ vents reveal ecosystem effects of ocean acidification. Bremerhaven (Germany) 27-30 September, 2010.

10 Trotter J, Montagna P, Rodolfo-Metalpa R, McCulloch M, Silenzi S, Reynaud S, Martin S, Ferrier-Pagés C, Gattuso JP. Quantifying the pH "vital effect" in the temperate shallow-water coral Cladocora caespitosa: validation of the boron seawater pH proxy. ISRS "International Society for Reef Studies" European Meeting; Wageningen (Germany) 13-17 December, 2010.
9 **Rodolfo-Metalpa R** & Ferrier-Pagés C. The future of Mediterranean scleractinian corals in a warming Mediterranean Sea. ASLO "Aquatic Sciences Meeting 2009", Nice (France) 25-30 January, 2009.

8 **Rodolfo-Metalpa R**, Martin S, Ferrier-Pagès C, Gattuso J-P. Are all zooxanthellate scleractinian corals sensitive to ocean acidification? 11th PSI "Pacific Science InterCongress", Tahiti (French Polynesia) 02-06 Mars, 2009.

7 Hall-Spencer JM & **Rodolfo-Metalpa R**. Natural CO₂ vents reveal ecological tipping points due to ocean acidification. Third International Symposium on the Ocean in a High-CO₂ World; Monaco 6-9 October, 2008.

6 **Rodolfo-Metalpa R**, Martin S, Ferrier-Pagès C, Gattuso J-P. Effects of elevated pCO₂ and temperature on calcification and photosynthesis of the Mediterranean coral *Cladocora caespitosa*. 11th International Coral Reef Symposium, Fort Lauderdale, Florida (USA) 7-11 July, 2008.

5 Hall-Spencer JM, Fosså H, Rüggeberg A, Fine M, **Rodolfo-Metalpa R**. Effects of ocean acidification on temperate corals. 4th ISDSC "DeepSea Coral Symposium", Wellington (New Zealand) 1-5 December, 2008.

4 **Rodolfo-Metalpa R**, Martin S, Richard C, Reynaud S, Gattuso J-P, Ferrier-Pagès C. Response of the Mediterranean coral *Cladocora caespitosa* to elevated pCO₂ and temperature. Change in Aquatic Ecosystems: Natural and Human Influences. Plymouth (UK), 4-6 July, 2007.

3 Martin S, **Rodolfo-Metalpa R**, Reynaud S, Ferrier-Pagès C, Gattuso J-P. Response of Mediterranean benthic coralline algae and corals to elevated pCO₂ and temperature. Surface Ocean CO₂ Variability and Vulnerability Workshop, UNESCO, Paris (France) 11-14 April, 2007.

2 **Rodolfo-Metalpa R**, Richard C, Allemand D, Ferrier-Pagès C. Effect of long-term exposure to elevated temperature on two Mediterranean corals. 13th Ocean Sciences Meeting; Honolulu, Hawaii (USA) 20-24 February, 2006.

1 **Rodolfo-Metalpa R**, Richard C, Allemand D, Ferrier-Pagès C. Effect of sub-lethal increases in temperature on two Mediterranean corals. ISRS "International Society for Reef Studies" European Meeting, Bremen (Germany) 19-22 September 2006.
# 2. PREAMBULE ON MY RESEARCH ACTIVITIES, PROJECTS AND RESEARCH HISTORY

## 2.1. Contribution to my field

I have a well-established track record of laboratory and field studies into the effects of climate change on various life stages of key marine species such as algae, seagrasses, foraminiferans, sponges, corals, bryozoans and molluscs.

- In the framework of EU FP7 EPOCA (European Project on Ocean Acidification), I performed the first long-term laboratory study on the effect of ocean acidification (OA) on Mediterranean corals and found that one Mediterranean coral was surprisingly resilient to the pH levels projected for 2100 (Rodolfo-Metalpa et al. 2010 Biogeosciences).

- I have pioneered the use of submarine CO₂ vents to investigate the responses of benthic communities to lower pH conditions providing the first studies of the effect of OA on ecosystems. This study was published by Nature and quickly became a keystone reference with natural CO₂ vents now frequently used to study the effects of OA.

- In 2009 I became a Prince Albert II Foundation independent PI based at the International Agency of Atomic Energy (AIEA, Monaco). My project (http://www.fpa2.com/projet-bassin-mediterraneen-numero-63.html) characterized the effect of climate change on key-calcifying organisms such as corals, bryozoans, mussels and limpets exposed to CO₂ vents. This research (Rodolfo-Metalpa et al. 2011 Nature Climate Change) revealed the mechanisms of resistance to OA across a range of taxa.

- I participated in the EU FP7 project MedSeA (Mediterranean Sea Acidification under a changing climate) helping to predict the effect on habitats formed by macroalgae, deep-sea corals and vermetid gastropods. Several studies have been published so far, and others are in preparation. Among the most relevant, Milazzo, Rodolfo-Metalpa et al. (2014) Scientific Reports is the first study reporting on the detrimental effect of acidification on a key reef-forming vermetid; Rodolfo-Metalpa et al. (2014) Global Change Biology assessed the coral ability to acclimatize to future warming; Rodolfo-Metalpa et al. (2015) Global Change Biology assessed cold-water corals resistance to acidification; and Garilli, Rodolfo-Metalpa et al. (2015) Nature Climate Change explained the physiological mechanism allowing some molluscs to survive to past extinctions caused by ocean acidification.

- In 2013 I was appointed to a permanent position (Research Scientist, first category) at IRD (Institut de Recherche pour le Développement) in New Caledonia. Here, among other research projects, I am using the CO₂ vent system of Papua New Guinea and the extreme sites of Bouraké and Palau as natural laboratories to study coral reef responses to ocean climate change. Thanks to two ANRs projects, and other Labex Corail and AFD grants, I have benefited from the necessary funds to investigate those natural laboratories, establish a high number of international collaborations, and publish several studies on the subject.

## 2.2. Recognitions from others

My research includes the first experiment on adaptation responses of marine organisms to OA and on the CO₂ dose required that brings about coastal ecosystem tipping points. The Nature paper has been cited >1500 times so far across a range of scientific disciplines and my first Nature Climate Change is expected to take a similar trajectory (>500 citations). Following the publication of these articles, the originality of these results and the uniqueness of the approach used have been reported in several scientific journals, on TV, newspapers and web blogs. The work has been used to help understand the fate of marine organisms at inter-governmental workshops (e.g., **Unesco, Surface Ocean CO₂ Variability and Vulnerability Workshop**, Paris April 2007; **IPCC workshop on Impacts of Ocean Acidification on Marine Biology and**
*Ecosystems*, Okinawa January 2011; **CBD** joint expert review on the *Impacts of ocean acidification on marine and coastal biodiversity*, Montreal, October 2011).

During a 5-year postdoc research period, I demonstrated that I could obtain competitive funding: I obtained a grant to carry out an independent project at CO2 vents (Prince Albert Foundations) and other travel and conference awards. My actual permanent position as a research scientist allows me to build, around the acquired expertise, a competitive team and a technically advanced laboratory. For that, during the period 2014 - 2020, I have been able to attract funders to my research projects (as PI) for a total of 2,200K€.

## 2.3. Research history

My main topic has always been the effect of climate change, with particular attention to OA, on a series of calcifying marine organisms, especially corals. My approach in doing that has been classic in my early studies, i.e., using lab-based experiments, and innovative during the following career since I pioneered the use of natural laboratories where climate change-like conditions naturally characterized the environment. I have published 71 papers so far; 5 before my PhD; and 8 during my PhD. During my post-doctoral career, 41 out of 58 papers I have published were based on natural laboratories simulating future conditions, while 14 used a lab-based approach.

A few early steps of my scientific background may explain the dynamic of my research and the continuity in the main theme that has characterised these last 23 years. In 1997, when I started my M.Sc at the Research Centre ENEA S. Teresa, and during the following four years of research, my supervisors CN Bianchi, A Peirano and C Morri introduced me to the world of scientific research. I performed my first experiment in aquaria, participated in some oceanographic cruises, performed my first coral transplantations in the field, and produced data for the first 5 papers. During my PhD at the Centre Scientifique de Monaco under the supervision of C Ferrier-Pagés and D Allemand, I continued the topic I was studying in Italy during my pre-PhD research period, i.e., the effect of warming on Mediterranean corals. This topic was enlarged to the emerging theme of OA thanks to the collaboration with J-P Gattuso from the laboratory CNRS LOV in Villefranche-sur-mer. From my PhD, I published 8 papers.

During two post-docs with Prof J-H Spencer at the University of Plymouth, which started immediately after my PhD, I was lucky to participate to EPOCA, and MedSeA, the two first European Research projects on OA. This has greatly helped and boosted my career and scientific curiosity. With J-H Spencer, I have pioneered the use of submarine CO2 vents to investigate the response of temperate benthic communities to low pH conditions, providing the first studies on the effect of OA at the ecosystem scale. Natural CO2 vents, where continuous emission of near-pure CO2 from below the seafloor alters the seawater chemistry of the surrounding seawater, provide a powerful experimental tool to study the ability of species to acclimatize/adapt to future acidification levels. Since our first paper in 2008, CO2 sites have been frequently used as natural analogues to study OA effects on single species and/or entire communities. This approach has strongly characterised my future research during which I have repeatedly used such special sites, and discovered others. For the first time I have described the CO2 vents of Ischia and Vulcano (Hall-Spencer, Rodolfo-Metalpa et al., 2008 Nature; Boatta et al., 2013 Mar Poll Bull), the Gulf of California deep vents (Prol-Ledesma et al., 2016, Nature Comm), and Ambitle Island (Pichler et al., 2019, Mar Poll Bull). I have worked at the most famous CO2 vents in Normanby (PNG, Fabricius et al., 2011) and White Island (NZ, Brinkman and Smith, 2014), as well as at the Nikko Bay in Palau (Reimer et al., 2023, Coral Reefs). In addition to CO2 vents, I discovered a unique site where the three main environmental parameters of climate change, i.e., OA, warming and deoxygenation combined in a single site,
the Bouraké semi-enclosed bay in New Caledonia (Camp et al., 2017, Scientific Reports; Maggioni et al., 2021 Biogeosciences). Most of my research projects from 2017 have been done in Bouraké, and most of my future projects will use this awesome natural laboratory. During this first 26-year-long scientific trip, I have established strong and friendly collaborations with several colleagues working on the same topic. These collaborations were consolidated in the ICONA consortium, "International CO2 Natural Analogues Network (ICONA)", a core-to-core international collaborative project that aims at using natural analogues to study the effects of OA on coral reef ecosystems. This project has allowed us to work, in 2023 at Nikko Bay (Palau, Micronesia), likely the most emblematic semi-enclosed bay where corals have adapted to 1°C warming and the OA level expected for the future.

The main advantage of using natural laboratories to study the effects of ongoing climate change is that results are ecologically relevant, although some limitations must always be kept in mind. Indeed, most conclusions of impacts of OA (as well as other factor such as heat-shock stress, deoxygenation, nutrient enrichment, etc) on organisms and consequent extrapolations to the ecosystem level, stem from short-term laboratory experiments on individual organisms. These experiments are certainly informative, as they enable us to identify the effect of one or a few variables, but in isolation they are unable to account for the implications of species' capacity for acclimatization (and adaption) in natural environments. As a consequence, the time exposure to future global change conditions, and the experimental condition itself, become a central issue in improving our understanding of marine organisms' capacity for acclimation to future conditions. Laboratory experiments are also not ecologically realistic since, for instance, they remove the effects of species interactions, natural supply of nutrition, and environmental fluctuations in the main environmental parameters. In addition, the quasi-totality of studies has neglected the role of adaptation since they tested only within-generation responses to global change and during short-term exposure to stress. Exceptions are studies using short-term life-span organisms such as the coccolithophores, copepods and polychaetes, sea urchins and fish or using breeding experiments on sea urchin larvae.

The use of natural labs has certainly helped my recruitment at IRD, UMR ENTROPIE in October 2013 after a brief postdoc period, and fostered the achievement of two ANR grants: CARiOCA - Coral reef acclimatization to ocean acidification at CO₂ seeps, and PNG-Vents - Using volcanic CO₂ vents to assess coral reefs ability to acclimatize and adapt to ocean acidification and warming. The two consecutive projects allowed my team to perform 7 fieldwork missions in PNG to study corals, sponges and fish around the acidic seawater at CO₂ vents but also allowed me to collaborate with several experts on this topic, therefore forming a network of scientists, all experts on the effect of climate change on coral reefs and associated species. Natural laboratories have been the perfect tool for me to test either specific mechanisms or scale up to habitat-level responses to environmental changes of a series of organisms.

After my PhD in 2007, most of my research has focused on corals (32 out of 58 papers), including shallow water temperate, tropical species, as well as cold water corals, and anemones, but also other taxa (19 out of 58) including bryozoans, coralline algae and molluscs, for which my contribution has been relevant, and forams, coccolithophores, algae, fish, and sponges, which have been investigated thanks to external collaborators with specific expertises. Most of the time I have investigated the effect of OA, sometimes in combination with warming, on calcifying reef species metabolic rates and symbiosis (e.g., photosynthesis, respiration, tissue parameters, etc), but my main interest has always been about their ability to calcify since it is the most obvious and uniform effect across most of the organisms studied in the OA context (Kroeker et al. 2013; IPCC 2015). For that, over my studies, calcification has been investigated
using classic, state-of-the-art techniques such as buoyant, alizarin staining, transplantation etc (39 out of 58), but also using isotopic tracers and geochemical techniques (13 out of 58).

I realise that most of the time my findings have been against the consensus about the effect of OA on metabolism in general, i.e., I have rarely highlighted a dramatic effect of low seawater pH levels on the coral calcification rates despite most studies demonstrating up to 83 % reduction in the coral calcification ability (Langdon et al. 2005). Why that? I decided to elaborate on this question in this HDR manuscript, which is intended to demonstrate that I have worked during my career around a fundamental question, and try to shed light and solve it, which probably is not the case! So, if there is one question I have always tried to answer, it is “Why OA should affect some fundamental metabolic mechanisms, such as calcification, as initially claimed?” Are my divergent results merely the fruit of my desire to stand out from the mainstream? Are they just casual results? or even worse, has the effect of OA been somewhat overstated?

In attempting to answer the question “Why should corals care about ocean acidification?” I will briefly contextualise the danger posed by ocean acidification to corals, trace a synthesized description of the main studies about the effect of OA on coral calcification, revise the main points which have built the consensus about the effect of OA on coral reefs, and to what extent this consensus has been modified from the first studies made around 20 years ago. I will report only studies regarding the effect of OA on calcification.

In this HDR manuscript, I will not enter into the details of the complex chemical and physical mechanisms that govern coral calcification, still to be fully revealed. Indeed, it was not the goal of my research. Then, I will first resume to what extent my studies have contributed to answering the above-mentioned question, and then briefly give some description of what I think have been milestone papers for my research. In doing that, I will describe only studies I have published over the past 20 years focusing on the effect of OA on the calcification ability of reef calcifying species, mostly corals. My intention was not merely to list one paper after another, because although they are listed quite in chronological order, actually they have been incremental in the original scientific question: “Why coral calcification should care about OA?” Those studies have been conducted both using a classic lab-based approach, i.e., maintaining organisms in aquaria under artificial conditions, as well as using natural analogues of future conditions, i.e., natural laboratories where environmental conditions already mimic future scenarios. Since I have been at the forefront of this last approach, and most of my research has been done using those natural labs, I’ll give a particular emphasis to such a kind of approach that has deeply marked my career. Finally, I will describe my actual research and future perspective, including the project I presented to the last ERC CoG call in 2021, which remains in my opinion an innovative project; and an invaluable challenge to realise in my career.
# 3. WHY SHOULD CORALS CARE ABOUT OCEAN ACIDIFICATION: TOWARD A REVISED CONSENSUS?

Ocean acidification (OA) is one of the main threats to marine habitats likely causing changes in biodiversity and ecosystem function within this century. Ocean acidification might affect various physiological parameters at different stages of the animal life history, from their reproduction, through larval phases and adult growth. Calcifying species, such as scleractinian corals which harbour a large part of the world's ocean biodiversity, apparently will be the most affected by OA since their calcification and dissolution rates seem related to seawater carbonate chemistry. Studies show a decline in the net calcification rates as a result of decreasing pH and carbonate ions concentration, and an increase in the dissolution rate of carbonate skeletons. However, the consensus on the projected hindering of species calcification ability is in contradiction with several past and recent findings showing that some calcifiers do not seem to be affected by OA. These divergent results have clearly shown that our actual knowledge of biological response to OA is extremely limited and that some assumptions we have used so far might be inaccurate to predict species response to OA.

In this report, the consensus of the response of coral calcification rates to acidification, early erroneous assumptions and misconceptions will be revised in the light of recent milestone contributions which impose an adjustment of our current understanding on coral calcification responses face to OA.

## 3.1. Building the consensus on the ocean acidification threat

Since the Industrial Revolution, atmospheric CO₂ levels have increased and have nearly doubled since then. The projections for future OA indicate that if CO₂ emissions continue to increase at the current rate, the pH of the surface ocean is estimated to decrease by around 0.4 units by the end of this century compared to pre-industrial levels (Jiang et al. 2023). This rate of acidification has been unprecedented in the past 65 million years, posing significant challenges for marine ecosystems and organisms.

About a third of anthropogenic CO₂ emissions has been absorbed by the oceans, driving the process of OA during which absorbed CO₂ transforms into carbonic acid, increasing the concentrations of H⁺, bicarbonate (HCO₃⁻), and dissolved carbon dioxide (CO₂), while lowering carbonate (CO₃²⁻) concentration, the calcium carbonate saturation state (Ω), and seawater pH (Orr et al. 2005). Ω is the ratio of the ion concentration product ([Ca²⁺] x [HCO₃⁻]) to the solubility product (K'sp) for the mineral aragonite which compose the coral skeleton.

Coastal regions, where human activities and inputs are concentrated, can experience additional localized pH issues (He & Silliman 2019). Factors such as nutrient runoff from agriculture, sewage discharge, and industrial pollution can exacerbate acidification in these areas. In addition, coastal waters naturally experience pH variability due to various factors, including tidal cycles, temperature changes, freshwater input from rivers, and upwelling events. These natural fluctuations can occur over diurnal (daily) and seasonal time scales. Increased nutrient inputs from agricultural activities and wastewater discharge can stimulate the growth of algae and phytoplankton, leading to excessive organic matter production. In turn, the subsequent decomposition of this organic matter consumes oxygen and releases CO₂, exacerbating localized future acidification.

The notion that the increase in atmospheric pCO₂ forecasted for the 21st century has a negative effect on coral reefs, was born during the late 1990s when a decrease in Ωara, and therefore CO₃²⁻ was demonstrated to negatively affect coral calcification (Smith and Buddemeier 1992). Gattuso et al. (1999) wrote the first exhaustive review of calcification and photosynthetic mechanisms on corals from the perspective of the relatively new threat to coral reef health. Based on IPCC estimates of pCO₂ increases and the few studies published until 1999 on this
topic, a calcification rate decrease by 10% from 1880 to 1990 was measured and a further decrease by 9 to 30% between 1990 and 2100 was predicted. These estimations were extrapolated from four studies on coralline algae, and three others on coral reef mesocosm (references in Gattuso et al. 1999). Although the Authors supported the hypothesis that calcification is controlled by seawater Ω<sub>ara</sub> and that experimental evidence suggests a severe drop in calcification rates in the future, they admitted that experimental data were not sufficient to provide an accurate prediction of future scenarios. Further, Kleypas et al. (1999), Gattuso and Buddemeier (2000), and Langdon (2000) used the same dataset on projected changes in the seawater Ω<sub>ara</sub> to predict a 17-35% decrease in the reef calcification over the next century. These Authors concluded that “*several studies have shown that calcification rates of reef-building corals and coralline algae are depressed by increased levels of CO<sub>2</sub>...*,” and that “*the decreased saturation state could result in reduced calcification rates*”.

Based on these experimental pieces of evidence scientists posted an alarming message for the future of coral reefs face to OA (Hoegh-Guldberg et al. 2007; Erez et al. 2011). Rates of calcification in scleractinian corals will drastically decrease by the end of this century as a result of reduced seawater carbonate concentration due to a doubling of preindustrial levels of atmospheric CO₂ (Kleypas et al. 2006). Furthermore, a 40 to 83% decline in reef calcification (Langdon et al. 2005) and a global shift in coral reefs from net accreting to net dissolving was predicted by the year 2065 (Silverman et al. 2009). For example, a study conducted by Jury and Thomas (2001) examined the effects of reduced pH on the calcification rates of *Porites astreoides*, a common Caribbean coral species. The researchers found that decreasing the pH of seawater leds to a significant decline in coral calcification rates. Cohen and colleagues (2009) found that reduced pH levels reduced the calcification rates of the reef-building coral *Porites astreoides*. Similarly, a study by Albright et al. (2018) demonstrated that lower pH levels decreased calcification rates and altered the skeletal structure of the coral *Porites cylindrica*. The impacts of pH on coral calcification are further compounded by other stressors, such as increasing seawater temperature and nutrient pollution, which can interact synergistically with ocean acidification. However, evidence for a consistent relationship between Ω<sub>ara</sub> and coral calcification was not always well supported and disagreed with the findings of Lough and Barnes (2000) who did not detect a long-term (1903-1979) decrease in calcification in cores of the long-lived coral *Porites* collected on the Great Barrier Reef. In addition, Atkinson et al. (1999) reported long-term high growth rates for 57 species of coral growing in relatively high-nutrient, very low-pH seawater (ca. 7.6) which indicates that corals can continue to calcify under extremely low CO<sub>3</sub><sup>2-</sup> concentrations.

After more than 20 years of intense experimental research on the field of OA, it seems that the consensus on the effect of OA on coral reef calcification has persisted until very recently, even though several studies have demonstrated past misconceptions and method biases, and several species have been shown to resist the harmful effects of OA. For example, cold-water corals thrive well and without decreasing their calcification rates near the aragonite saturation horizon (i.e. Ω<sub>ara</sub> = 1) (references in Rodolfo-Metalpa et al. 2015); some temperate species seem to calcify at normal or even higher rates at acidified seawater (e.g. Rodolfo-Metalpa et al. 2011; Ries et al. 2009); several studies have shown high variable responses of tropical species, including no effect (e.g. Houlbréque et al. 2012; Reynaud et al. 2003).

Is coral calcification impaired by OA? Will coral reefs dissolve as claimed? The increasing knowledge acquired during the last years, especially on the calcification mechanisms, demonstrated that commonly reported predictions on the detrimental effects of OA – reduced calcification and enhanced dissolution – may have to be revised (Roleda et al. 2012).

Here, I revise the consensus on the effect of OA on the coral calcification and dissolution rates in the light of some milestones studies (e.g. Venn et al. 2012, 2011; McCulloch et al. 2012a,
2012b; Comeau et al. 2015a, 2015b, 2014a, 2014b, 2013; Jokiel 2011a, 2011b; Jokiel et al. 2014; Murillo et al. 2014; etc) which have greatly improved our knowledge on coral reef responses to OA, and very recent metanalyses (e.g. Leung et al. 2022; Connell and Leung 2023). Throughout this study, the deleterious effect of warming will be only partly mentioned but it is clear that predicted temperature increases during this century have the potential to greatly affect coral reefs in combination or not with acidification. While the exact effects of OA on the main physiological mechanisms are still subject to debate, there are no doubts that coral reefs are particularly sensitive to temperature increases and that warming will be lethal for most species (e.g. Rodolfo-Metalpa et al. 2011).

## 3.2. Effect on calcification rates at the organismal scale. Is coral calcification impaired by OA?

One of the main physiological expenditures affected during a period of stress that defines a coral's health status is its somatic growth. Coral calcification is a vital process that plays a crucial role in the growth and survival of coral reefs. It refers to the ability of corals to build their calcium carbonate skeletons, which form the structural foundation of the reef. Maintaining a high rate of coral calcification is crucial for the growth and resilience of coral reefs, providing them with structural integrity and protection from physical disturbances. Calcification is tightly linked with photosynthesis (i.e., light-enhanced calcification; Gattuso et al. 1999), and to the efficiency of certain Symbiodiniaceae to translocate photosynthetic compounds to their hosts (Cooper et al. 2011). Understanding the role of the different carbonate forms ($CO_2$, $HCO_3^-$, $CO_3^{2-}$) on calcification mechanisms, and metabolism in general, is essential to establish informed predictions about future OA impacts on coral reefs. A poor understanding of the calcification mechanisms involved has impaired progress in this debate. Even the fundamental question of which source of inorganic carbon is favoured by corals for their mineralization remains partially unanswered (Allemand et al. 2011). The process of coral calcification involves the uptake of dissolved inorganic carbon (DIC) from the surrounding water, conversion of DIC into bicarbonate ions, and precipitation of calcium carbonate crystals within the coral's tissues (Cohen and McConnaughey 2003). This regulation is critical for the proper functioning of cellular processes involved in calcification, and is strongly related to the coral's ability to maintain a range of internal pH. Evaluating the roles of $CO_2$, $CO_3^{2-}$, and $HCO_3^-$ in supplying dissolved inorganic carbon (DIC) for corals calcification poses a significant challenge to progress in this field of investigation, notably in terms of describing calcification mechanisms upon which further studies of OA can be based, and determining the response of corals to OA. To date, several hypotheses have been proposed concerning the origin of the DIC necessary for calcification but they are still matters of controversy. It seems that most of the studies have taken for granted that the formation of $CaCO_3$ would result from the combination of $Ca^{2+}$ and $CO_3^{2-}$ dissolved in seawater. Since calcification and dissolution rates of marine species are related to seawater carbonate chemistry, empirical relationships between $\Omega_{ara}$ or [$CO_3^{2-}$] and $CaCO_3$ deposition have been used to predict future calcification and dissolution rates in response to OA (e.g. Erez et al. 2011; Kleypas et al. 2006; Kleypas et al. 1999). Since the $Ca^{2+}$ concentration is near conservative in seawater, $\Omega_{ara}$ is largely determined by [$CO_3^{2-}$] in seawater, which in turn is one of the building blocks of calcium carbonate (Gattuso and Buddemeier 2000).

Nevertheless, at least three main reactions might explain the formation of $CaCO_3$:

$$Ca^{2+} + CO_3^{2-} = CaCO_3 \quad (1)$$

$$Ca^{2+} + HCO_3^- = CaCO_3 + H^+ \quad (2)$$

$$Ca^{2+} + CO_2 + H_2O = CaCO_3 + 2H^+ \quad (3)$$
It is rare that bicarbonate ions (eq. 2) or carbon dioxide (eq. 3), which can move freely through cell membranes and are available from the photosynthetic process, have been proposed as a substrate for calcification in OA studies. Re-examination of the past literature shows that eq. 1 may practically not contribute to the CaCO₃ formation (Allemand et al. 2011) because the [CO₃²⁻]/[HCO₃⁻] ratio at physiological pH (between 7.5 and 9.0 pH units) is extremely low (Venn et al. 2009).

It is known that corals accrete their CaCO₃ structures in a fluid-filled medium called subcalicoblastic medium (Allemand et al. 2011) underlying the calicoblastic epithelium which is the calcifying tissue. In the organism's calcifying space, an elevation of more than 1 pH unit compared to ambient pH makes the calcification possible (Allemand et al. 2011; Trotter et al. 2011; Venn et al. 2011). Internal pH is elevated by the activity of the antiport Ca²⁺/H⁺ ATPase, which transfers Ca²⁺ to the site of calcification in exchange of protons. The exchange of H⁺ drives the equilibrium toward CaCO₃ formation converting HCO₃⁻ to CO₃²⁻ and thereby increasing the saturation state (Ω) in the calcifying fluid (Cohen and McConnaughey 2003). Using different approaches, Trotter et al. (2011), McCulloch et al. (2012a), and Venn et al. (2011) have nicely shown that some tropical, temperate and deep-sea corals maintain strong gradients of pH between the surrounding seawater and the internal calcifying coral space over a range of seawater pH values, including end-of-century projected values. Therefore, carbonate precipitation in calcified structures does not directly originate from water carbonate but is generated or modulated via several reactions from HCO₃⁻ and/or CO₂ in the alkaline compartment at calcification rates. The way the two other DIC forms (CO₂ and HCO₃⁻) might be delivered to the calcification sites and used to calcify has been extensively discussed (Allemand et al. 2011; Jokiel 2011a, 2011b), and their direct effects on the calcification rates have been proven for some species (e.g. Comeau et al. 2013; Jury et al. 2010; Marubini et al. 2008; Herfort et al. 2008; Schneider and Erez 2006).

In a first attempt to tentatively separate the contribution of the various components of the seawater carbonate system, corals were incubated in seawater where carbonate parameters were manipulated by adding acid, bases or both. Some studies (e.g. Schneider and Erez 2006) showed an enhancement of coral growth after an increase in DIC, suggesting that the ambient DIC concentration of seawater may limit the calcification rates of hermatypic corals. For that, Marubini et al. (2008) separately manipulated the concentration of HCO₃⁻ and pH to investigate the physiological mechanisms underlying coral calcification. They found that *Stylophora pistillata* grew faster in bicarbonate-enriched seawater independent of pH conditions (pH 7.6 to 8.2) suggesting that calcification was C-limited. Although CO₃²⁻ is the ion species deposited at the calcification site, it seems clear that such deposition at the site of calcification depends on biologically mediated HCO₃⁻ transport from the external medium. Corals can obtain the carbon for calcification from HCO₃⁻ in the seawater or from metabolically produced CO₂ (Furla et al. 2000). Carbonic anhydrase located in the cytoplasm of the coral converts HCO₃⁻, which is the majority of host intracellular DIC (Venn et al. 2009), into CO₂ and HCO₃⁻, which are then used for photosynthesis and calcification, respectively. Manipulating HCO₃⁻ concentration at constant pH (8.2) on the corals *Acropora* sp. and *Porites porites*, Herfort et al. (2008) confirmed the role of HCO₃⁻ in stimulating calcification both on corals exposed to light and in the dark, not only by stimulating photosynthesis. However, in this experiment, HCO₃⁻ and CO₃²⁻ covaried and their effect could not be separate. This was done by Jury et al. (2009) who found that the calcification of the coral *Mandracis mirabilis* not only was unaffected by OA but was enhanced by adding HCO₃⁻. Comeau et al. (2013) designed a manipulative experiment to test the roles of HCO₃⁻ and CO₃²⁻ on the calcification of the coral *Porites rus* and the crustose coralline algae *Hydrolithon onkodes* in the light and dark. The Authors demonstrated that both forms are involved in calcification, and new conceptual models of calcification were proposed.
By measuring skeletal boron geochemistry, Allison et al. (2014) confirmed that for the massive *Porites* spp. $HCO_3^-$ mainly contributes to the DIC pool used during calcification.

Therefore, calcification does not seem to always depend on the concentration of seawater carbonate ions as previously assumed (reviewed in Erez et al. 2011). Roleda et al. (2012) pointed out that although a comprehensive understanding of coral calcification mechanisms was puzzling at the beginning of the OA research era, there was enough knowledge to assert that calcification in corals is biologically modulated and not simply the combination of $CO_3^{2-}$ and $Ca^{2+}$ in a seawater medium. It seems that pioneering OA studies overlooked state-of-the-art knowledge to explain, more simplistically, how the decrease in the $CO_3^{2-}$ and $\Omega_{ara}$ resulted in the observed decrease in coral calcification rates.

So why several studies have shown that at the scale of individual organisms, coral calcification rates were affected by high or even moderate decreases in pH, $CO_3^{2-}$ or $\Omega_{ara}$? Is coral calcification impaired by OA?

It is important to note that studies have used a range of physical and chemical conditions which might have resulted in different results, and that the sensitivity of corals to OA is more species-specific than previously assumed. Pandolfi et al. (2011) and Edmunds et al. (2012) reviewed the possible reasons for this high variability among studies and species. In general, they found that the variance in the response to high $pCO_2$ may be attributed not only to specific physiological mechanisms and differences among coral species (fast vs. slow-growing corals, e.g. Rodolfo-Metalpa et al. 2010), but also to the limitations of experimental methodologies (e.g. manipulation of seawater chemistry), the experimental duration, the experimental setup (water flux, light irradiance, e.g. Comeau et al. 2014b), and the origin of the samples (e.g. samples collected at reef flat where pH variations vary more in than other habitats, Comeau et al. 2014c). A recent meta-analysis exercise on the sensitivity of coral calcification to changes in $\Omega_{ara}$ (Chan and Connoly 2013) showed that responses to OA may be less severe than previously suggested. Their analysis suggests that large between-study variability was not explained by different manipulations of seawater chemistry, nor by the coral calcification rates or the culturing irradiance levels. Interestingly, the Authors found that studies measuring calcification via the alkalinity anomaly method (see below) found significantly larger decreases in calcification than studies using buoyant weighing, likely because the latter integrates both light and dark calcification, and it measures the whole coral growth.

To explain the paradigm between the observed reduction in calcification under conditions of increasing OA and the uncertain role of the different DIC components, Jokiel (2011a, 2013) did not investigate the coral calcification process as previous investigators did, therefore trying to understand which components corals require for calcification, but from the end of the process: which wastes corals need to remove to allow calcification. The Author formulated an elegant story, the “Proton flux hypothesis” where coral calcification is regulated by the net efflux rate of $H^+$ produced during the calcification process (equation 2), from the coral tissue into the surrounding water. Corals are separated from seawater by a boundary layer, called diffusion boundary layer (DBL), the strength of which influences $H^+$ removal, as well as other coral metabolic waste products. Since the increasing OA causes an increase in the seawater $H^+$ concentration, the gradient becomes less favourable reducing the flux of protons out of the coral and decreasing its calcification rate. The model reveals that $HCO_3^-$ is the substrate for calcification, obtained either directly from seawater or derived from mitochondrial $CO_2$ that is converted by carbonic anhydrase into $HCO_3^-$. The Proton flux hypothesis was further integrated into the “two-compartment proton flux model” (Jokiel 2011b) in which the Author unified existing theories and paradoxes of coral biology in a milestone contribution. Since photosynthetic and calcification processes compete for available inorganic carbon in corals (Gattuso et al. 1999), they developed a morphology which separates the zone of rapid
calcification (ZC) from the zone of rapid photosynthesis (ZP), with ZC situated between the ZP, the diffusion boundary layers (DBL) and the external seawater. Within the DBL, the transport of ions and gases to and from the organism's surface is limited by diffusion, which is, in turn, dependent on the concentration gradient across the DBL. Among the advantages for coral metabolism, this spatial arrangement allows a rapid translocation of the fixed-carbon energy supply from the ZP to the ZC and the efflux of protons out of the coral, therefore regulating calcification and photosynthesis. Furthermore, Jokiel (2013) used data from Comeau et al. (2013) to test the Proton flux hypothesis asserting that $CO_3^{2-}$ and $\Omega_{ara}$ have no basic physiological meaning for calcification. Instead, calcification correlates well with the ratio of DIC to proton concentration ([DIC] : [H⁺]).

A step forward is the contribution of Hohn and Merico (2015) which improved previous mathematical models for coral calcification suggesting that the combination of transcellular ion transport and paracellular pathway is the most likely mechanism for coral calcification at a reduced energetic cost. Their model indicates that the transcellular pathway would favour the carbon transport across the coral cells via free diffusion of $CO_2$ over cell membranes, while paracellular diffusion would assure seawater exchange at the calcifying fluid directly. Their conceptual model calculations reveal that the majority of carbon (ca. 98%) enters the calcifying fluid via diffusion of the metabolic $CO_2$, while only 0.8% is transported actively as bicarbonate. Carbon dioxide which can subsequently react with $H_2O$ to form $HCO_3^-$ and H⁺ (catalyzed by the enzyme carbonic anhydrase) could therefore be an alternative inorganic carbon source for calcification in particular taxa.

Hohn and Merico (2015) also evoked a key question regarding the cost of compensating pH differences under OA conditions, therefore in maintaining or not calcification rates (Erez et al. 2011; Pandolfi et al. 2011). This is highly energy consuming (Allemand et al. 2011) with an estimated increase in the metabolic cost of ca. 10% per 0.1 pH unit decrease in seawater pH (McCulloch et al. 2012b). If calcification becomes energetically more costly under elevated $pCO_2$ due to a decreased $\Omega_{ara}$, then the extra energy needed to maintain calcification might be drawn by increasing feeding rates and/or drawing upon energy reserves. This assumption is particularly important for aposymbiotic corals such as cold-water and deep-sea corals which are uniquely heterotrophic. However, in general, the few studies performed so far did not find decreases in the cold-water coral calcification rates, neither their lipid reserves nor an increase in their respiration rates (reviewed in Rodolfo-Metalpa et al. 2015). McCulloch et al. (2012a) calculated that a decrease in seawater pH from 8.1 to 7.7 units has an extra cost for symbiotic reef-building corals to calcify, which however is insignificant (<1%) when related to the total energy produced by their photosynthesis. Accordingly, Schoepf et al. (2013) found that the energy reserves of four tropical species did not decline with increasing OA during experimental manipulation in aquaria, suggesting that energy reserves do not play a role in sustaining calcification under OA conditions, or that the increased energetic costs of maintaining calcification under OA are relatively insignificant.

### 3.3. Effect on the reef net community calcification. Will coral reefs dissolve by future OA levels?

Among the few studies used in a first attempt to define the emerging threat of OA on coral reef calcification, some were conducted at the community scale, therefore considering the whole habitat formed by species and substrates (references in Langdon et al. 2010, and Gattuso et al. 1999). These studies used experimental mesocosms where a portion of coral reef was reproduced in large aquaria and the conditions expected in the future were simulated. It is important to note that such a kind of approach has been fundamental to assessing the effect of climate change on coral reefs since it might provide critical information regarding the potential interactive effects of the major biological and environmental parameters in a more realistic and
ecologically relevant setup. Mesocosm experiments integrate a panel of organisms, natural environmental parameters and their variability, as well as food availability and types of substrates to simulate the natural environment. Although its importance, a limited number of studies have been conducted so far at the ecosystem scale using experimental mesocosms (e.g. Comeau et al. 2015a, 2015b; Andersson et al. 2009), likely due to the methodological complexity in reproducing the structure and diversity of benthic communities and in maintaining such systems. While results from experiments conducted on individual corals have shown variable responses ranging from no effect (e.g. Reynaud et al. 2003) to negative (e.g. Erez et al. 2011), and even positive effects (e.g. Rodolfo-Metalpa et al. 2011), in general, studies on the effect of OA on the whole reef community have consistently demonstrated negative effects on the coral net community calcification rates (NCC). For instance, Leclerq et al. (2000) found that the NCC at the Monaco mesocosm was a linear function of Ω<sub>ara</sub> and that it decreased by 21% in response to acidification. In a long-term study, although individual corals remained healthy and actively calcified, Andersson et al (2009) found a decrease in NCC and net dissolution in response to OA levels expected at the end of this century.

Mesocosm experiments have greatly strengthened the consensus in the OA literature that calcification is inhibited through a reduction in seawater [CO₃²⁻], and thus Ωara (Kleypas et al. 2006). Similarly, measurements of the daily natural variability in carbonate chemistry over coral reefs have strengthened the use of Ωara as one of the main factors controlling the reef NCC, and results have been used to get reliable global-scale predictions. Metabolic and physical processes taking place in a reef, at least for those located in semi-closed lagoons and shallow back reef habitats alter the carbonate chemistry of the surrounding seawater sometimes reaching levels expected in the future. Therefore, the relationship between natural variability in seawater carbonate chemistry and reef NCC can be used to predict future responses to OA (Shaw et al. 2012).

In general, results from natural environments, experimental mesocosms and model prediction have shown that there is a strong positive coupling between seawater Ω<sub>ara</sub> and the reef NCC (Silverman et al. 2009). Under expected acidification levels, daily NCC will be significantly lowered relative to the rate and deposition of CaCO<sub>3</sub> at ambient conditions, and net dissolution at very low Ω<sub>ara</sub> will be most likely, which means that NCC of many reef systems could become negative because the production of CaCO<sub>3</sub> will be exceeded by its dissolution (Langdon 2000; Andersson et al. 2009). Based on the observed relationship between Ω<sub>ara</sub> and reef calcification, on average, community calcification was predicted to decrease by 60% per unit decrease in Ω<sub>ara</sub> with values ranging from about 15% to 130% (references in Andersson and Gledhill 2013).

But, if it is true that coral calcification is not always affected by OA as previously thought, why
several studies have shown that at the reef community scale coral calcification rates were
affected by a decrease in CO3²⁻ or Ωara? Will coral reefs dissolve as claimed?

It is important to note that NCC rates have been estimated based on measurements of carbon
chemistry in open-flowing water over natural reef communities, mesocosms or flumes. This
method assumes that, although a community cannot strictly be described as exhibiting a
metabolism per se, it is possible to describe the integrated performance of the ecosystem in
terms of the rates at which metabolically relevant chemical species (e.g. DIC, A<sub>T</sub>) are modified
within the overlying water column. Recent estimates of NCC based on community metabolism
from different reef environments yield a broad range of rates, from 22 mmol CaCO<sub>3</sub> m<sup>-2</sup> day<sup>-1</sup>
to 331 mmol CaCO<sub>3</sub> m<sup>-2</sup> day<sup>-1</sup>. These large discrepancies reflect differences in the reef types,
hours of the day, seasons, reef structures and their compositions. The first potential pitfall of
this method is that the reef calcification rates are calculated from changes in water-acid base
status through the alkalinity anomaly technique (Chisholm and Gattuso 1991) which assumes
that for every two moles of total alkalinity (A<sub>T</sub>) consumed one mole of CaCO<sub>3</sub> is produced. The
reverse reaction occurs during dissolution. Although this technique has been validated for short-
term incubations (Chrisholm and Gattuso 1991) and low-size organisms (Langdon et al. 2010),
interfering metabolic and acid-base regulation processes cast some doubt on the absolute rates
determined. This is especially true when this technique is used to measure the calcification of
organisms with large biomasses which excrete significant metabolic wastes, likely affecting A<sub>T</sub>
values and thereby reef NCC. Other coral reef processes such as organic matter production from
photosynthesis, anaerobic diagenesis in sediments and microbial activity, as well as nutrient
transformation (references in Murillo et al. 2014 which recently revised the method) might
affect A<sub>T</sub>. More importantly, when applied to assess the whole reef calcification, this approach
integrates the calcification response of all species composing the reef and includes the effect of
dissolution of all the biogenic substrates and their bioerosion to the reef NCC. Indeed, at the
community scale, it is almost impossible to separate the contribution of dissolution from net
calcification, because it is impossible to measure the reef gross calcification. In contrast, at the
organism scale the effect of dissolution on the net coral calcification can be measured by the
difference of the gross and the net calcification rates. Labeling experiments with the radio-
isotope ⁴⁵Ca in aquaria is the only technique likely to provide measurements of gross
calcification, and it has been applied for the first time to test OA effect on the calcification of
two temperate corals: *Cladocora caespitosa* and *Balanophyllia europaea* (Rodolfo-Metalpa et
al. 2011). While the net calcification rates of *C. caespitosa*, which has skeletal parts that are
exposed to the surrounding seawater, significantly decreased at pH<sub>T</sub> < 7.8 and the coral suffered
severe skeletal dissolution, *B. europaea*, which has a skeleton completely covered in tissue, was
not affected. By measuring both the net and the gross calcification, it was possible to show that
remarkably both corals were able to maintain gross calcification rates at pH levels expected to
the end of this century and beyond. These results suggested that coral tissue protects the
underlying skeleton from dissolution under OA conditions as recently confirmed for the reef-
building coral *Stylophora pistillata* (Tambutté et al. 2015). By measuring the gross calcification
of *S. pistillata* held in aquaria at pH<sub>T</sub> 7.8, Houlbréque et al. (2012) found no effect of OA. This
is in agreement with Cohen and Fine (pers. comm.) which did not find differences in the net
and gross calcification rates of nubbins of *S. pistillata* held for 16 months at pH<sub>T</sub> 7.6. In another
study, Rodolfo-Metalpa et al. (2015) did not find differences in the net, gross calcification and
dissolution rates of the cold-water coral *Desmophyllum dianthus* held at pH<sub>T</sub> values between
8.1 and 7.7 units. These are only some examples showing that some corals can calcify under
OA conditions and that the dissolution of the exposed skeletal structures might be more
important than the effect on calcification, according to the protective role played by tissues in
preventing dissolution. Although it would be speculative to extrapolate organism-based
responses to whole-ecosystem responses, it is evident that skeletal dissolution and bioerosion
have an important contribution to the observed decrease in reef NCC subjected to OA
conditions. This has been shown by Shamberger et al. (2014), who found relatively high coral
cover and diversity on Palau's low-pH bay reef. Furthermore, Barkley et al. (2015) showed that
while coral calcification rates at this site were unaffected by OA, coral bioerosion increased 11-
fold as pH decreased.

Because reefs are made of diverse habitats with various mineral compositions and benthic
communities, calculating NCC from AT variations does not allow distinguishing which reef
habitats contribute the most to the final net dissolution (e.g. sediments, corals, macroalgae).
This method, for instance, cannot decipher the importance of calcification vs. dissolution rates
of the different species and substrates forming a reef. It is likely that the calculated tipping point
at which reef carbonate production shifts from net calcification to dissolution (e.g., Andersson
et al. 2009; Silverman et al. 2009) is mainly dependent on the contribution of the different reef
habitats. Reefs with predominant fractions of sediment, coral pavement (i.e. cemented CaCO3
substratum covered by an assemblage of turf and coralline algae) or macroalgae are important
components of reef systems (e.g. lagoon, backreef, forefront). Under natural conditions, and especially at night, when metabolic respiration drives a decrease in the seawater Ω<sub>ara</sub>, reef dissolution is indeed most attributable to processes occurring within the carbonate sediments. Under low Ω<sub>ara</sub> conditions, Andersson et al. (2009) found that most of the dissolution most likely occurred in the thin sediment layer present at the bottom of their mesocosm. Murillo et al. (2014), measuring the NCC of different reef components in flume experiments, suggested that the total alkalinity technique was only valid for coral-dominated environments. When sediments and algae were included, the ratio between A<sub>T</sub> and Ca<sup>2+</sup> deviated from the theoretical 2.0 value, which means that these other substrates largely contributed as sources of A<sub>T</sub>. They found that for a mixed community (i.e. algae, sediment and corals) the A<sub>T</sub> method will estimate a lower net calcification than for a coral-only community due to A<sub>T</sub> recycling. The importance of the sediment fraction in the observed decrease of the reef NCC rate has been further confirmed (Eyre et al. 2014). In other flume experiments, NCC was reduced by 59% and 49% (Comeau et al. 2015a and Comeau et al. 2015b, respectively) under OA conditions. In these studies, the sediment and the reef pavement dissolution explained respectively ca. 50% and 78% of the observed decreases. Using NCC data available from 233 locations including 183 reefs around the globe, Cornwall et al. (2023) investigated in a meta-analysis study the net carbonate production rates and responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change by 2050 and 2100. The authors found that future scenarios will cause a net decrease in calcification and an increase in reef erosion. These declines are largely the result of reduced coral cover from bleaching events rather than from the direct impacts of ocean warming or acidification on calcification or bioerosion.

Another fundamental caveat in predicting responses of whole reef ecosystems to a change in the seawater chemistry is that the assumed linear relationship between the NCC and Ω<sub>ara</sub> is actually altered by the reef photosynthesis and respiration rates, which greatly vary among reefs and all over the daily cycle. Strong relationships between the net reef community production (NCP) and NCC have been found (e.g. Jokiel 2014; Andersson and Gledhill 2013; Cyronak et al. 2013; McMahon et al. 2013) leading to a diel hysteresis pattern in the NCC *versus* Ω<sub>ara</sub>. Jokiel (2014) measured during daily cycles seawater carbonate chemistry, NCC and NCP in mesocosms containing mixed components of the reef (coral, algae and both the components). Results showed that changes in seawater Ω<sub>ara</sub> are a consequence of changes in both the reef NCC and NCP rather than a driver of NCC. These results have important implications because if further confirmed, they would invalidate most of the predictive models which assume that the reef NCC is controlled by seawater Ω<sub>ara</sub>.

**3.4. Putting my contribution into the OA debate**

What has been surprising during at least the first 15 years of research into the OA debate is that studies showing “different” responses to OA, thereby deviating from the consensus, have timidly increased in number but substantially they did not contribute to changing the message. This is not to say that OA won’t affect coral reef calcification, structure integrity and diversity, but the effect will likely be less dramatic than it has been predicated so far. Curiously, during this intense phase of research, which was uniquely focused on the effect of one factor, OA, we all forgot that the future will be the combination of at least two factors, OA and warming, the latter for sure the most dangerous.

My research initially contributed to the consensus but quickly deviated toward a more nuanced effect or no effect at all. Indeed, two first studies (Hall-Spencer et al. 2008; Martin et al. 2008) evaluated the intertidal and seagrasses habitats exposed for centuries to acidified seawater emitted by shallow water CO<sub>2</sub> seeps in the Mediterranean Sea. Our data revealed a shift in communities, with the reduction of important organisms such as sea urchins, and limpets and the absence of scleractinian corals under lowered pH conditions. Coralline algae, which play a
crucial role in seagrass ecosystems, disappeared in acidified conditions, leading to a significant reduction in epiphytic calcium carbonate. These findings underscored the sensitivity of calcareous organisms to elevated pCO₂ levels, and in agreement with most of the studies published from mesocosm and lab-based experiments, highlighted the potential consequences of OA at the ecosystem scale. I might honestly recognise, thanks to my research on other CO₂ seeps in Papua New Guinea, that one of the most common criticisms in using such a natural system to investigate the effect of OA is that the effect is exacerbated by the CO₂ variability. Indeed, we found that the “tipping point” in the pH causing the shift between calcareous to non-calcareous algae around pH 7.83, but it is possible that the shift was caused by the high variability of the pH at the study site with values lower than 7.4. These undersaturated seawater likely caused the observed dissolution of shells and other carbonatic tests. That said, on the one hand, this study was honest in the data presentation and its limits, in the other hand it was the first evidence of the danger that OA might be for a whole ecosystem. For that reason, other CO₂ seeps, almost in the tropic have been further used in the same manner reinforcing the consensus on the effect of OA on coral reefs (see Fabricius et al. 2011, for an example).

However, in Rodolfo-Metalpa et al. (2010), my first study on the effect of OA level predicted to 2100, alone or in combination with a +3°C temperature, did not give the expected results, since we found that both short- and long-term incubation do not fundamentally affect the calcification rate (and the metabolic rates) of the Mediterranean coral *Cladocora caespitosa*. This result was surprising. It introduced a new dimension to the topic by demonstrating that not all organisms respond in the same way to increased pCO₂ levels. Some species, particularly those with slow growth rates, may not exhibit reduced calcification rates. It emphasized that factors like seasonal temperature changes play a significant role in determining the response of organisms to OA, and the impact may not be as widespread as previously thought. In the meantime we finalised this paper, the most famous study by Ries et al (2009) was published, showing similar results on the coral *Oculina arbuscula*, and other calcifiers. One of the biases of my study was that the experimental setup was done to study Mediterranean CCA living at 20-30 m depth, and the temperatures we tested were relative at this depth and not to the upper surface where MHWs threat ecosystems. While *C. caespitosa* lives from 40 m depth to the first 5 m depth, most of them are common in the first 10-15 m and, therefore subjected to higher temperatures (and increasing with the ongoing warming), especially during summer. For that the higher temperature tested in my study, 25.4°C was realistic only for the deep population, and useless to test the combined effect of warming and OA. Although the bias, this study was my first research showing that OA responses are more complex than commonly thought.

In a further study (Rodolfo-Metalpa et al. 2010, Marine Ecology) I highlighted the importance of considering the simultaneous effects of acidification of temperature, this time using an unexpected heat wave during an *in situ* transplantation experiment. Live and dead colonies of the branched bryozoan *Myriapora truncata* were transplanted to normal (pH 8.1), high (mean pH 7.66, minimum value 7.33) and extremely high- CO₂ conditions (mean pH 7.43, minimum value 6.83) at CO₂ vents off Ischia island (Tyrrhenian Sea, Italy). The net calcification rates of live colonies and the dissolution rates of dead colonies were estimated by weighing after 45 days (May-June 2008) and after 128 days (July-October). Data helped to assess if high CO₂ levels affect bryozoan growth and survival differently during moderate and warm water conditions. It seems that *M. truncata* can maintain its calcification rate when exposed to quite low seawater pH but at normal seasonal temperature, but when combined with high temperature, their combination can lead to the mortality of organisms that initially seemed resilient to OA. This suggests that climate change factors, including rising temperatures, can exacerbate the challenges posed by OA and need to be taken into account in predicting future responses.
My previous study using the bryozoan *M. truncata* opened the question about the ability of some species to calcify even at very low pH but, at the same time potentially have their skeleton dissolved because of the lower aragonite (or calcite) saturation state value of seawater. In the previous study, I calculated the gross calcification simply by adding the amount of skeleton being dissolved to the measured net calcification, which is quite inaccurate. In contrast, in Rodolfo-Metalpa et al. (2015, Nature Climate Change) I measured both net and gross calcification on transplanted corals, mussels and limpets. This study explored the protective mechanisms employed by some calcifying organisms to limit skeleton dissolution when exposed to acidic conditions. It showed that certain species when exposed to high CO₂ levels, could calcify and grow even faster than under normal conditions, and suggest that this is a mechanism they use to repair their skeleton. However, as pH levels further drop, the dissolution of shells and skeletons became a risk, underscoring the significance of tissues and organic layers in protecting these structures from corrosive seawater. Our transplantation experiments show each of the species we examined was able to calcify (i.e. gross calcification) and some calcified faster at pH values well below those projected for global surface waters by 2100. Their tolerance to acidification depended on their ability to maintain this protection at elevated levels of CO₂. We showed that although some organisms can up-regulate calcification at lowered carbonate saturation states, they rely on protective organic layers to avoid dissolution. Therefore, projected levels of OA are likely to increase erosion of unprotected biogenic carbonate structures. Worryingly, the coastal calcifiers that we transplanted along natural CO₂ gradients were more vulnerable to the effects of OA when the water was warmest, indicating that OA will likely exacerbate the mass benthic mortality events that have been recorded with increasing frequency in the warming Mediterranean Sea. These corals showed no significant impact on calcification rates under expected pCO₂ levels for 2100. The findings emphasize the importance of considering multiple factors, including temperature and protective mechanisms, when assessing the vulnerability of calcifying organisms and ecosystems to future changes in ocean chemistry.

In another study (Rodolfo-Metalpa et al. 2015, Global Change Biology) we provided insight into the resilience of cold-water corals to OA. There were concerns that cold-water corals are even more vulnerable as they live in areas where aragonite saturation (Ωara) is lower than in the tropics and is falling rapidly due to CO₂ emissions. However, most of what we know about cold-water coral (CWC) and deep-sea coral (DSC) metabolism was measured during artificial conditions in the lab, since *in situ* data on the physiological response of CWC to environmental variations are logistically hard to get. This certainly has hindered our ability to project their fate in the face of rising CO₂ levels. As a result, it is possible that most of the data published on the effect of OA on those corals could merely be biased by the nutritional diet given during the experiment in aquaria since nothing is known about their diet, and because it has been proven that nutrition affects the performance of organisms (“Fed corals are happy corals”: source F. Houlbreque). So, in this experiment, three species of CWC where maintained and artificially fed in aquaria, and their net and gross calcification measured. To test for the effect of these artificial conditions, the only way was to transplant the corals in their natural environment, both at normal and low seawater pH. The only site that was suitable for this kind of experiment was the Ischia CO₂ seeps site where I transplanted one species of them (i.e. *Desmphyllum dianthus*). Since Ischia is an unnatural site for these corals, because it is too shallow, I transplanted the same species (plus two more) also at 300 m depth, where the pH was normal and most importantly the corals could benefit of a natural nutrition. I found that the three coral species had no change in their gross calcification under OA conditions. The species maintained at 300 m had the same calcification rate as its counterpart maintained at low pH in Ischia at very low pH conditions. This study confirmed that one must consider the results obtained in aquaria with caution, especially when the nutrition of the studied organism is unknown. Furthermore, both
aquarium and *in situ* results confirm that acidification does not have a dramatic effect on coral growth.

Further fieldwork observations made at the CO₂ seeps in Vulcano strengthen the hypothesis that some species already know how to counteract future levels of OA. Near the CO₂ seeps at very low pH water, and buried in the sediment where the pH is even lower than in the surrounding seawater, I found shells of two species of tiny gastropods quite dissolved by the undersaturated seawater but still alive. In this study (Rodolfo-Metalpa et al. 2015, Nature Climate Change) we showed that i) two gastropod species adapted to acidified seawater at shallow water CO₂ seeps were smaller than those found in normal pH conditions; ii) they consistently grew less than at the control pH, and iii) they had higher mass-specific energy consumption but significantly lower whole animal metabolic energy demand. These physiological changes allowed the animals to survive, although at a reduced size, and to maintain the calcification mechanism necessary to partially repair shell dissolution. These observations of long-term chronic effects of increased CO₂ levels support the hypothesis that OA contributed to past extinction events and suggest that some species could adapt through dwarfing (i.e., the “Lilliput effect”) conferring physiological advantages as the rate of CO₂ emissions continues to increase.

Although most of my studies showed limited effect of OA on some species, i) they were valid only for Mediterranean species; ii) they mixed observations made at unique sites with high pCO₂ variability and during lab-based, unnatural experiment conditions. Therefore my research, although innovative, could not undermine the consensus formed around the tens of amazing studies produced at the magic Normanby Island CO₂ seep, the natural laboratory that Katharina Fabricius discovered and used with professionalism and intelligence for years, in collaboration with top specialists on the effect of OA on corals and other reef organisms. The consensus was that OA would flat coral reefs and only a few winner species would survive. My findings were therefore seen as curious and interesting deviations from the truth.

From 2016 to 2019, in the framework of the project CARIOCA, I visited 3 times the famous CO₂ seeps site studied by K. Fabricius, and 4 times a new CO₂ seep in PNG, Ambitle Island. I have been slow in publishing the data we collected showing i) the higher primary production of corals from these sites (Bisceré et al. 2019; Meunier et al. 2021); ii) the winning association of the corals living at CO₂ seeps with particular Diazotroph strains (Geissler et al. 2021) conferring higher thermal resistance to the host (Meunier et al. 2022). Furthemore, data collected in collaboration with M. Hoogenboom and T. Shlesinger (still to be published) demonstrated that OA only partially decreased the coral diversity and abundance and suggested that the low diversity found in Normanby could be site-specific, not a general response of coral reef to OA. While I failed to measure the growth rate of corals transplanted at these sites, using geochemical techniques such as the boron signature in the skeleton of several coral species we showed (Comeau et al. 2022, Global Change Biology) that corals from the CO₂ seeps were always able to maintain higher pH at the calcification rate, therefore to calcify, than corals from the control reef. This ability seems to give an ecological advantage to the coral since casually the species that showed a higher ability to maintain high internal pH were also the most abundant at the CO₂ seeps. In addition, by measuring seawater pH at high frequency, at each site and during each mission, suggested that corals at Normanby are subjected to sudden changes in pH when compared to Ambitle and they were exposed to a very large variability in seawater pH. Although pH averaged similarly, Normanby was more variable than Ambitle, and likely caused some of the effects previously reported.
In New Caledonia, thanks to the discovery of Bouraké, where the geomorphology of the site causes seawater to vary in pH, oxygen, temperature and other environmental and chemical parameters, I have conducted several experiments to test the effect of OA in combination with other factors on the coral calcification, and not only. The most surprising evidence at such a site is that the healthy corals living there seem to grow well at pH which regularly varies from normal to 7.3 (in total scale)! This posed the question of how they can continue to calcify at pH and saturation state for aragonite so low. In the first study, performed with colleagues from UTS (Australia; E. Camp and D. Suggett) we first characterized the environmental conditions during 2 weeks and the first measurements of the physiological response of three species of corals. Using the alkalinity technique (short incubations) we found that corals from Bouraké calcified, although less than the reference ones, and respired more, likely as a consequence of the higher heterotrophic ability compensating the higher cost of calcification. After that, I performed a reciprocal transplantation of four species between 2 stations in Bouraké and 2 outside, as a reference (unpublished) and found, using the buoyant weight technique (i.e., difference in the coral weight in the long term) that i) corals from Bouraké always grew more than their counterparts from the reference site; ii) corals from the reference sites grew better when inside in Bouraké; iii) corals from Bouraké suffered at the reference sites. These preliminary (1-year experiment) data suggested that both the Bouraké population has adapted to the environmental conditions, and that the environmental condition itself can favour their growth. To test for the hypothesis that the higher heterotrophic condition of the Bouraké lagon caused the observed higher growth rate, therefore better investigating the potential of adaptation of these corals, and also to test the effect of seawater pH variability, previously suggested to increase the corals resilience, with my PhD Clement Tanvet, we performed a 100 days experiment in aquaria testing uniquely the effect of pH (constant and variable) on 3 coral species from both Bouraké and Reference. All corals were fed in the same manner. This study confirmed my previous data on transplanted corals and added a new crucial piece to the puzzle. Indeed, i) it excludes that heterotrophy is the main factor causing the high growth in Bouraké since both populations received the same level of nutritional diet in aquaria; ii) it sheds light on the ability of corals from Bouraké to calcify at a reference site (i.e. at normal pH). Indeed, while in my first transplantation I measured a decrease in calcification of corals when transplanted from Bouraké to the reference, the same corals maintained in aquaria at normal pH (reference conditions) grew even faster; iii) it confirms that Bouraké corals have in some ways adapted to the extreme conditions since they always maintained higher growth, irrespecting the seawater pH; iv) it confirms (after preliminary results by Camp et al. 2020) that one of the mechanisms of adaptation was their diverse Symbiodiniaceae hosted in their tissue. These results were further confirmed during the PhD of Cinzia Alessi who performed a transplantation of 2 coral species from 2 reference sites in Bouraké.

If during both *in situ* observations and lab-based experiments I found variable and sometimes limited effects of OA on adult corals, one of the bottlenecks of the reef status is their ability to reproduce, settle and continuously form persistent reefs. Healthy and functional coral reef communities directly depend on their ability to release larvae, and to their recruitment success that, in turn, depends on the abundance of their preferred settling substrate i.e., crustose coralline algae (i.e., CCA). Ocean acidification has been predicted to greatly affect different stages of this natural and complex mechanism. My first study in this research field was on the ability of the coral *Pocillopora damicornis* from Bouraké to reproduce, settle and grow in the laboratory under acidified conditions (Rodolfo-Metalpa et al., unpublished). This story has been rejected by the same reviewers three times, in three different journals. I left it in a corner but it seems we have been quite unlucky. Indeed, the major concern was that we compared offspring performances of colonies from two different sites, Bouraké and one control reef, for which we do not know the whole peak planulation timing and, therefore the optimal larval phenotypes
versus suboptimal larvae. The reviewer concluded with this encouraging sentence “Further the conclusions of this manuscript come from comparing apples to oranges at the two different sites given the well-documented impact of larval release times on larval phenotype”. Well, *Pocillopora damicornis* release larvae from October to Mai, and clearly at different rates from site to site, colony to colony and year to year. It will take years for me to have a valid dataset on the peak planulation timing for the two studied sites. We aimed to assess if life-long preconditioning to a contrasting fluctuating environment can change the offspring tolerance to OA and warming during 28 days of exposure to manipulated *pCO₂* (500 and 1,500 ppm) and temperature (26 and 29°C) in a full-factorial mesocosm experiment. At the end of the incubation, we measured the offspring post-settlement survival, lateral growth and budding of new polyps. Likely, this study said something the reviewer did not like, that is the lack of differences between the two populations, one adapted to extreme conditions, and one from a normal fringing reef. Our measurements confirmed the negative effect of high *pCO₂* on the lateral growth and budding rates of recruits, and we found no evidence that individuals from a more variable environment gain in fitness performances and tolerance to future-like conditions. The skeletal microstructure of recruits from both environments was basically unaffected, although the elevated *pCO₂* tested. We insisted on the observation that reproductive output was more important in corals from Bourake because colonies produced enormously more planulae than colonies from the reference. But we were right as we demonstrated in the study made by my PhD student Clement Tanvet on the effect of the Bouraké extreme conditions on CCA coverage settled on artificial ceramic tiles, and coral recruitment rates (Tanvet et al. 2021). In this study, we assessed the effects of extreme and fluctuating environmental conditions at Bouraké on the abundance of CCA, algal turf, and coral recruits. To address this objective, we obtained CCA-covered tiles from a forereef and moved them to Bouraké and two fringing coastal reef sites with distinct environmental conditions. Based on the reported effect of OA on CCA cover and coral recruitment rates, we hypothesized that in Bouraké both will be heavily affected by the extreme conditions. While we confirmed previous findings on the sensitivity of CCA to OA, since they decreased in coverage in Bouraké, surprisingly, we found that coral recruitment rates in Bouraké were higher than at both the two fringing reefs. Our findings contradicted results from several OA laboratory-based experiments and reinforced the idea that coral recruitment is possible even under extreme conditions and that, particularly for the Bouraké site, corals may have adapted to such variable and extreme environmental conditions. Finally, my PhD student Cinzia Alessi measured during three years the spawning timing and fecundity of *Acropora tenuis*, *Montipora digitata*, and *M. stellata* from Bouraké and three reference sites located at variable distances from Bouraké (Alessi et al. 2023 in prep.). Interestingly, no reduction in the number of eggs per fecund polyp or proportion of fecund polyps was observed in Bouraké compared with the reference sites, in any species. However, *A. tenuis* and *M. digitata* had smaller eggs in Bouraké compared to reference sites, with possible repercussions on the development of early life-history stages and coral fitness. Cinzia also measured less coral recruits on ceramic tiles settled in Bouraké during the years 2020-2022, which is in contrast to our previous findings in Tanvet et al. (2021) during the year 2017-2019. Likely, this difference was due to the effect of the three consecutive years of La Nina in New Caledonia and the consequent massive coral mortality of corals in Bouraké, but this is another story.....
**3.5. Conclusions and perspectives**

OA might affect coral calcification, mainly on species that cannot regulate their internal pH and facilitate the deposition of CaCO₃ as some others do. OA might have an effect on sediments and reef skeletons causing their dissolution, and this will be more important if corals will die and the reef will degrade in the future. OA might affect other taxa than corals, some will take advantage of the high CO₂ levels, such as macroalgae, maybe sponges; others will be strongly affected such as plankton species. OA might have an indirect effect on the abiotic parameters that in turn regulate the complex exchanges between corals metabolism and the surrounding environment. OA might interact with other environmental parameters and combine toward a synergistic negative effect. The most prominent example is its combination with high temperature, which itself seems to be the causative factor of the next coral mass extinction.

Having said that, in light of my research and some recent analyses of the scientific output on this topic, I think that the effect of OA on calcification will be marginal on the survival of corals in the face of ongoing global warming.

An attentive reader will certainly have noticed that this memorandum, which intends to answer the question "Why should corals care about ocean acidification?" is already old. I started writing about these considerations back in 2016. Year after year I added articles without ever finding the courage to publish what should be a food for thought paper, perhaps out of fear, perhaps out of opportunism, since I have been working on this for almost 20 years.

My research has been centred on the effect of OA on corals and other organisms, especially corals, especially concerning their ability to calcify. Of course, I have studied other organisms, of course not only calcification, of course using both aquaria and natural laboratory setups, but in the end my main research has been transplanting corals and measuring their calcification. I started this approach when the theme was very hot and the expectations were dramatic since the OA issue was expected to devastate corals. Fortunately, the effect has been demonstrated to be less important than previously announced and my studies have partially contributed to improving this research field. It is worth mentioning my research demonstrating the protective role of coral tissue (and periostracum on bivalves) in preventing skeleton dissolution. Another result I consider very important was the measurement of the gross calcification of corals (therefore excluding any contribution of skeletal dissolution) I did use the ⁴⁵Ca technique at IAEA. For the first time, it was demonstrated that corals can calcify at the same rates under OA conditions. But what has fundamentally pushed my understanding of the effect of OA on corals was to see that actually, corals can thrive in extreme environments such as at CO₂ seeps and other natural analogues such as Bouraké and Nikko Bay. The chance I had to use these special sites has certainly improved my knowledge of the resilience of various organisms to future conditions and suggested to us the potential mechanisms used by some species to face such extreme environments. Natural analogues are imperfect systems to test for the effect of future, even worse environmental conditions on resident, likely adapted organisms. Although the potential limitations, such as the variability of the abiotic parameters, the confounding effects of other unexpected parameters, the difficulty of replicating the sites, etc, natural laboratories are the best way to improve our ability to better test for coral resilience and adaptation to climate change. By using these natural laboratories, I furnished pieces of evidence that adaptation is
likely possible. What I could not demonstrate is if the corals that were able to survive simply acclimatized or adapted to the harsh conditions. In both cases, it becomes crucial to better understand if these mechanisms are rapid or take several generations to guarantee the coral's survival under future conditions. This becomes central to give some hope to the survival of coral species and whole reef ecosystems in a rapid climate change, and to cope with the rapid on-going change. My ERC project, which was not funded, addresses this issue. The aquarium facilities I am building in Nouville will be a great tool to boost this project, despite the lack of a consistent research fund. I'll do my best for that.

The project will focus on species that occur both inside (suboptimal environment) and outside (reference environment) the natural analogue of Bouraké. My ambition is to target at least five coral species that have developed and grown in these contrasting conditions throughout their entire life cycle, possibly over several generations (which is the big ambition of this project). A multi-disciplinary experimental approach will be used that quantifies variations in physiology, genetic makeup and functional traits (i) among species, (ii) within species using colonies collected inside and outside the Bouraké system to assess long-term acclimatization and/or adaptation; and (iii) among clones and genotypes using individuals reciprocally transplanted between environments. Reciprocal transplant experiments will be carried out using clonal fragments of multiple genotypes for each species to assess their level of acclimatization vs. genetic adaptation. This project will be the first to experiment with corals that have been exposed chronically, throughout their life, to suboptimal conditions (temperature, acidification, and deoxygenation) in a natural environment. Long-term transplantation of adult colonies and their recruits will be performed in situ during transgenerational experiments, providing robust evaluations of corals' capacity for phenotypic buffering and propensity for adaptation.

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Rodolfo-Metalpa R, Martin S, Ferrier-Pagés C, Gattuso JP. Response of the temperate coral *Cladocora caespitosa* to mid- and long-term exposure to pCO₂ and temperature levels projected for the year 2100 AD. Biogeosciences 2010; 289-300.

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# 4. PREVIOUS RESEARCH

## 4.1. Response of the temperate coral *Cladocora caespitosa* to mid- and long-term exposure to pCO₂ and temperature levels projected for the year 2100

Biogeosciences, 7, 289–300, 2010

www.biogeosciences.net/7/289/2010/

© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

<figure><img src="image_4.png" /></figure>

Biogeosciences

# Response of the temperate coral *Cladocora caespitosa* to mid- and long-term exposure to pCO₂ and temperature levels projected for the year 2100 AD

R. Rodolfo-Metalpa<fnref n="1" />, S. Martin<fnref n="2" />, C. Ferrier-Pagès<fnref n="3" />, and J.-P. Gattuso<fnref n="2" />,<fnref n="4" />

This study started in 2006, during the last year of PhD but was finally published in 2010. This detail is important to correctly place the study in the growing OA literature as it was one of the first long-term experiments in aquaria testing for the combined effect of OA and OW across seasons, the first on Mediterranean corals. The study was performed in collaboration with Sophie Martin and Jean-Pierre Gattuso in the framework of the first EU project on OA, EPOCA, led by J-P Gattuso. This study and this collaboration have terribly boosted my career.

## Introduction to the topic

At the beginning of the century, the scientific community started realizing that atmospheric CO₂ partial pressure (pCO₂) already increased by 32% between 1880 and 2005 (280 vs. 379 µatm), and a further doubling was expected by the end of this century (IPCC 2007). This finding was the fundamental recognition that anthropogenic CO₂ emitted to the atmosphere and absorbed by the oceans is causing the decrease in pH, CO₃<fnref n="2" />⁻ concentration and the related CaCO₃ saturation state (Ω) of seawater, a phenomenon called ocean acidification (OA). Rising atmospheric pCO₂ is also causing an increase in global sea temperature with an expected additional average warming of ca. 3°C (Mc Neil and Matear 2007).

Although limited in 2006, some studies already showed that coral calcification rate is largely controlled by the degree of saturation of seawater for aragonite (Ωara; see review by Kleypas et al. 2006), which varies with latitude (Orr et al. 2005). As a result, coral calcification is expected to decline dramatically in the future, raising widespread concerns about the future of our oceans in a high-CO₂ world (e.g. Hoegh-Guldberg et al. 2007). Several studies on the effect of OA on fast-growing tropical corals show that calcification could decline by 0 to 56% under a doubling of pCO₂ alone (Kleypas et al. 2006) or in combination with a +3°C increase in temperature (Reynaud et al. 2003, Anthony et al. 2008). In contrast, rates of photosynthesis are either not affected (e.g. Langdon et al. 2003, Reynaud et al. 2003, Schneider and Erez 2006, Marubini et al. 2008) or slightly increased (e.g. Langdon and Atkinson 2005) at the level of pCO₂ expected in 2100. The only study that investigated the effect of high pCO₂ on a Mediterranean coral, *Oculina patagonica* (Fine and Tchernov 2007) revealed a complete dissolution of the skeleton at pH 7.4, a value lower than the one expected in 2100.

<footnote n="1">¹International Atomic Energy Agency-Marine Environment Laboratories, 4 Quai Antoine Ier, 98000, Principality of Monaco</footnote>

<footnote n="2">²INSU-CNRS, Laboratoire d'Océanographie de Villefranche, B.P. 28, 06234 Villefranche-sur-mer Cedex, France</footnote>

<footnote n="3">³Centre Scientifique de Monaco, Principality of Monaco</footnote>

<footnote n="4">⁴UPMC University of Paris 06, Observatoire Océanologique de Villefranche, 06230 Villefranche-sur-mer, France</footnote>
## The experiment

We investigated in aquaria the effect of mid- and long-term exposure in an orthogonal experimental design with normal and elevated temperatures (T and T+3°C) and pCO₂ (400 µatm and 700 µatm) on calcification and photosynthesis of the Mediterranean zooxanthellate coral *Cladocora caespitosa* living at 20-30 m depth. Colonies were subject to (1) mid-term perturbations (1 month) in summer and winter conditions of irradiance and temperature, and (2) a long-term perturbation (1 year), mimicking the seasonal temperature changes (from 13.4°C to 21.7°C) and irradiance.

## Main results and discussion

While temperature is a critical environmental parameter controlling the physiology and calcification of *C. caespitosa*, an increase in CO₂ partial pressure, within the values expected by the end of 2100, did not significantly affect either their photosynthetic performance or the calcification rates. In this study, we confirmed previous findings on the effect of temperature on the calcification rates of *C. caespitosa* (Rodolfo-Metalpa et al. 2008b, c) and other temperate corals (e.g. Jacques et al. 1993, Howe and Marshall 2002). Rates of photosynthesis and

respiration were much lower in winter than in summer. Such reduction of photosynthetic activity could explain the difference in calcification rates between the two seasons. A 3°C increase in temperature during this winter period stimulated the coral metabolism, enhancing both photosynthesis and respiration. This stimulation was higher in summer and caused a significant increase in the calcification rates in agreement with the stimulating effect of temperature reported for both tropical (e.g. Clausen and Roth 1975) and temperate corals (Jacques et al. 1983, Howe and Marshall 2002).

At 22°C, a 3°C increase in temperature did not affect photosynthesis, symbiont biomass and calcification rates. The highest temperature slightly increased, albeit not significantly, the respiration rate.

<figure><img src="image_5.png" /><figcaption>**Fig. 4.1.1.** Net photosynthesis and respiration (a), and calcification rates measured in the dark and at the culture irradiance level (b) during the mid-term experiments (summer and winter) on *Cladocora caespitosa* exposed to the combined effect of normal (400 µatm) and elevated (700 µatm) pCO₂, and normal (T) and elevated (T +3°C) temperatures. Data are mean±s.e.m.</figcaption></figure>
These results suggest that the coral metabolism was at its upper-temperature limit. Photosynthesis does not seem to be greatly affected by OA in symbiotic corals (e.g. Langdon et al. 2003, Reynaud et al. 2003, Schneider and Erez 2006), likely because corals do not rely on dissolved CO₂ for photosynthesis (Gattuso et al. 1999).

Laboratory and mesocosm experiments have shown a common trend of decreased calcification rates with increased pCO₂ in reef-building corals (e.g. Gattuso et al. 1999, Langdon et al. 2005, Kleypas et al. 2006). It has been suggested that below a value of Ω<sub>ara</sub> = 3.3, corresponding to a pCO₂ level of around 480 μatm, calcification rates will approach zero (Hoegh-Guldberg et al. 2007; Silverman et al. 2009). In contrast, the temperate coral *C. arbuscula* maintained similar calcification rates for Ω<sub>ara</sub> ranging from 1.95 to 3.86 (pCO₂ values of 709-475 μatm), suggesting that this species may be resistant to an increase in pCO₂ in the range predicted for the end of the century. Similarly, Ries et al. (2009) reported that the calcification rate of the temperate coral *Oculina arbuscula* is unaffected by an increase of pCO₂ up to 840 μatm (Ω<sub>ara</sub> = 1.8). A drastic decrease in calcification was only found at a pCO₂ of 2240 μatm, corresponding to a Ω<sub>ara</sub> of 0.8. The low sensitivity of these two temperate corals to an increase in pCO₂ is at odds with the consensus on the negative relationship between pCO₂ and calcification of tropical corals (14 to 30% decrease by 2100; Gattuso et al. 1999, Kleypas et al. 1999).

Except for Ries et al. (2009) and the present study, all experiments carried out on corals used tropical, fast-growing species which grow up to 5 times faster than their tropical counterparts (Rodolfo-Metalpa et al. 2006).

Fast-growing corals likely need higher Ω<sub>ara</sub> than the slow-growing *C. caespitosa* or other temperate or cold species. It is conceivable that the requirement for carbonate ions of slow-growing corals is low and that the concentration of carbonate ions does not become limiting, even under high pCO₂ concentrations (ca. 700 μatm).

Although the negative relationship between calcification and pCO₂ is widely accepted for tropical corals and other calcifying organisms, temperate species may be less sensitive to an increase in seawater pCO₂. For the Mediterranean zooxanthellate coral *C. caespitosa*, the temperature is by far the main factor controlling the rate of calcification, even at the lower Ω<sub>ara</sub> values predicted for the end of the century for surface seawater in most regions.

<figure><img src="image_6.png" /></figure>

**Fig. 4.1.2.** Relationships between the mean calcification rates of *Cladocora caespitosa* measured during the long-term experiment and: (a) temperature regimes (range 13.3–25°C); (b) Ωaragonite. Open square 400T; closed square 400T +3; open circle 700T; closed circle 700T +3. The coefficient of the regression is reported as R2. Data are mean±s.e.m. (n=7).
## 4.2. Using volcanic CO₂ seeps to study the effect of OA in a natural setting

Our first description of the organisms inhabiting the CO₂ seeps in Ischia (Hall-Spencer et al. 2008) was the first that proposed this kind of approach. These naturally occurring sites, where CO₂-rich fluids seep into the ocean from subterranean sources, offer a unique and powerful platform to investigate the impact of elevated CO₂ levels on marine life, and to advance our understanding of ocean acidification's ecological consequences. Among the advantages, CO₂ seeps provide an ecologically realistic setting (i) where marine organisms have been exposed for long-term (ii) CO₂ levels and pH reductions that closely mimic those projected for future oceans. This extended observation period enables us to assess the chronic and cumulative impacts of OA on marine ecosystems, offering insights that shorter-term experiments cannot provide. (iii) CO₂ seep environments encompass a wide range of marine habitats, including coral reefs, kelp forests, and seagrass meadows, therefore allowing us to investigate how OA affects various ecosystems and their resident species, leading to more comprehensive insights. In addition (iv), because of the presence of multispecies, and natural abiotic and biotic conditions, responses are naturally driven not only by the OA parameters but also by the interaction of the natural environment and species, which facilitates a more holistic ecosystem-level assessment. Finally, CO₂ seeps not only simulate future conditions but also provide an opportunity to observe natural variations in CO₂ levels and pH due to tides, currents, and seasonal changes. This variation allows researchers to explore how different temporal patterns of acidification affect marine life.

### 4.2.1. The CO₂ seeps site of Ischia Island (Gulf of Naples, Italy)

Ischia is a volcanic Island in front of the Gulf of Naples, where the most famous Vesuvio volcano is still active. The area is quite active and Ischia in particular has been subjected to several earthquakes, the last in 2017. In 2007, Jason Hall-Spencer formed a team including me, and two students from the University of Plymouth. During 3 weeks of intense work, we started reporting the first data on the effect of OA on the local benthic ecosystem. The fieldwork was possible only thanks to the collaboration with the Stazione Zoologica Anton Dohrn and local scientists with whom this collaborative study was performed. Following this first study, we have continued to use this special site performing several transplantations, opening to new international collaborations and finally producing several studies, some of them published in high-impact factor journals. This has contributed to making the site and the local scientific station worldwide known where to perform experiments and test hypotheses on the effect of OA at different scales, from bacteria to the whole ecosystem.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.2.1.** Castello Aragonese (on the left), and CO₂ seeps in a *Posidonia oceanica* meadow at 2 m depth south, Ischia, in June 2009 (From Hall-Spencer & Rodolfo-Metalpa 2012).</figcaption></figure>
An area of CO₂ seeps occurs in shallow water (<10 m depth) around Castello Aragonese (40° 43.84' N, 13° 57.08' E) off Ischia island (Gulf of Naples, Italy) where the local marine flora and fauna has been the subject of detailed long-term studies by researchers at the Stazione Zoologica Anton Dohrn (reviewed by Cigliano et al. 2010). The vent gas comprises 90 – 95 % CO₂, 3 – 6% N₂, 0.6 – 0.8 % O₂, 0.2 – 0.8 % CH₄ and 0.08 – 0.1 % Ar (no sulphur). These vents are especially suitable for OA studies since they lack the toxic compounds (e.g. sulphur and arsenic) that characterise the majority of marine vents (Tunnicliffe et al. 2009; Couto et al. 2010; Karlen et al. 2010). In addition, the vents are at the same salinity (38 psu) and have the same total alkalinity (2.5 mEq kg⁻¹) and seasonal temperature fluctuations (13-28°C) as the surrounding surface waters off Ischia. The acidified areas encompass intertidal rock, infralittoral rock, infralittoral sediment, cave-dwelling communities and seagrass meadows allowing investigations into the consequences of acidification on a range of different coastal habitats. Two vent sites have been monitored (north and south Castello) since April 2007 to assess gradients in seawater pH, dissolved inorganic carbon, and the saturation states of calcite and aragonite.

<figure><img src="image_8.png" /><figcaption>**Fig. 4.2.2.** Map of CO₂ vents sites north and south of Castello Aragonese, off Ischia Island, Italy. Water chemistry has been monitored in rocky shore zones N1-N3 and S1-S3 as well as at transplant stations C, B1 and B2 and Posidonia monitoring stations P1-P4 by Hall-Spencer et al. (2008), Martin et al. (2008), Rodolfo-Metalpa et al. (2010) and Cigliano et al. (2010).</figcaption></figure>

From 2008, in the framework of consecutive postdocs at Plymouth University, The Prince Albert Foundation and the International Atomic Energy Agency, I participated in around 15 studies using this site. A first assessment of the biological effects of the Ischia vents quantified the abundance of dominant macroorganisms in intertidal and subtidal zones within and adjacent to the acidified waters (Hall-Spencer et al. 2008). Subsequently, the carbonate content and biodiversity of seagrass epibionts were evaluated along a pCO₂ gradient (Martin et al. 2008) as well and invertebrate recruitment was investigated during spring larval settlement using artificial collectors (Cigliano et al. 2010). A series of calcifiers were transplanted such as the corals *Cladocora caespitosa* and *Balanophyllia europaea*, the molluscs *Mytilus galloprovincialis* and *Patella caerulea* and the bryozoans *Myriapora truncata* and *Calpensia mirabilis* (Rodolfo-Metalpa et al. 2010; Rodolfo-Metalpa et al. 2011), as well as cold water corals (Rodolfo-Metalpa et al. 2015). The coral microbiome was assessed (Meron et al. 2012), as well as the photosynthetic and enzymatic capacity of sea anemones (Suggett et al 2012), the
geochemistry and shell structure of bivalves (Hahn et al. 2012), limpets (Langer et al. 2014, 2018), bryozoans (Rodolfo-Metalpa et al. 2010; Lombardi et al. 2011), corals (Trotter et al. 2011), boring microflora on coral skeletons (Tribollet et al. 2018) and sponge distribution (Goodwin et al. 2013).

### 4.2.2. The CO₂ seeps site of Vulcano Island (Sicily, Italy)

Volcano Island specifically Levante Bay has been actively investigated by Italian volcanologists since a very active volcanic area. Shallow submarine gas vents in Levante Bay, Vulcano Island (Italy), emit around 3.6 t CO₂ per day providing a natural laboratory for the study of biogeochemical processes related to seabed CO₂ leaks and ocean acidification. While this site was used first by Prof Marco Milazzo to study the effect of OA on benthic organisms, we joined his team in 2011 within the framework of the European OA project MedSea (P Ziveri PI). At Levante Bay, we found intense submerged seeps along the beach in Levante Bay, where dispersed underwater leaks cover a 130 x 35 m area at less than 1 m depth. This huge degassing acidifies most of the bay along a gradient of intensity.

We first characterised (Boatta et al. 2016) the most prominent geochemical parameters of the waters of the whole Levante Bay area (Fig. 1) and we provided a thorough assessment of the spatio-temporal variability of pH/CO₂ of the northern shore where our OA studies have begun, as this coastline is situated well away from the main vents but has acidified waters that form a shallow pCO₂ gradient along ca. 200 m of rocky habitats.

<figure><img src="image_9.png" /><figcaption>**Fig. 4.2.2.** The Levante Bay (Vulcano Island, Eolian archipelago) and the stations used to assess the bay pH and seawater chemical variation along the coast (Boatta et al. 2016).</figcaption></figure>

In collaboration with my friend Marco Milazzo, I performed two common garden transplantations, one using the Med corals *Cladocora caespitosa*, and *Astroides calycularis* (still to be published), and the second one (Milazzo et al. 2014) using cores of the reef-building gastropod *Dendropoma petraeum*, which forms the troittoirs in the intertidal zone. Other studies focused on metabolic and enzymatic activities in sea anemones (Ventura et al. 2016), coccolithophore diversity (Ziveri et al. 2014), fish reproduction (Milazzo et al. 2016), and gastropods calcification (Garilli, Rodolfo-Metalpa et al. 2015).
<figure><img src="image_10.png" /><figcaption>**Fig. 4.2.3.** Some of the species that were transplanted and measured at Ischia and Volcano CO₂ seeps.</figcaption></figure>

**4.3. Volcanic carbon dioxide vents reveal ecosystem effects of ocean acidification**

doi:10.1038/nature07051

nature

LETTERS

**Volcanic carbon dioxide vents show ecosystem effects of ocean acidification**

Jason M. Hall-Spencer¹, Riccardo Rodolfo-Metalpa¹, Sophie Martin², Emma Ransome¹, Maoz Fine³,⁴, Suzanne M. Turner⁵, Sonia J. Rowley¹, Dario Tedesco⁶,⁷ & Maria-Cristina Buia⁸

This study was the first I published as a postdoc. It is a milestone in this topic and has offered the scientific community a new approach to studying OA and revealing the effect of OA at the ecosystem level. It describes for the first time the CO₂ seeps in Ischia, which has rapidly become the centre of the universe for such a kind of research study attracting a lot of scientists, and media. The work has been used to help understand the fate of marine organisms at inter-governmental workshops (e.g. Unesco, Surface Ocean CO₂ Variability and Vulnerability Workshop, Paris April 2007; IPCC workshop on Impacts of Ocean Acidification on Marine Biology and Ecosystems, Okinawa January 2011; CBD joint expert review on the Impacts of ocean acidification on marine and coastal biodiversity, Montreal, October 2011, etc). This study has pioneered the use of CO₂ seeps and has reinforced the need to study OA in nature, not only in artificial systems, thus including interaction between species, natural nutrition and introducing the concept of adaptation, which had never before been addressed in the field of OA research.
Our understanding of how increased ocean acidity may affect marine ecosystems is currently very limited since almost all studies have been in vitro, short-term, rapid perturbation experiments on isolated elements of the ecosystem. Here we show the effects of acidification on benthic ecosystems at shallow coastal sites where volcanic CO2 vents lower the pH of the water column. Along gradients of normal pH (8.1-8.2) to lowered pH (mean 7.8-7.9, min 7.4-7.5), typical rocky shore communities with abundant calcareous organisms shifted to communities lacking scleractinian corals with significant reductions in sea urchin and coralline algal abundance. This is the first ecosystem-scale validation of predictions that these important groups of organisms are susceptible to elevated levels of pCO2. Seagrass production was highest in an area at a mean pH of 7.6 (1827 µatm pCO2) where coralline algal biomass was significantly reduced and gastropod shells were dissolving due to periods of carbonate sub-saturation. The vent sites revealed a suite of organisms that are resilient to naturally high levels of pCO2 and that OA may benefit highly invasive non-native algal species. Our results provide the first *in situ* insights into how shallow-water marine communities might change when susceptible organisms are removed due to ocean acidification.

<figure><img src="image_11.png" /><figcaption>**Fig. 4.3.1.** Variation in pH, abundance of limpets and cover of algae at CO2 vents south of Castello d'Aragonese. Data are from stations S1-S3, from 18 April to 9 May 2007. The percentage cover of calcareous (triangles) and noncalcareous algae (circles) is shown.</figcaption></figure>
## 4.4. Effects of ocean acidification and high temperatures on the bryozoan *Myriapora truncata* at natural CO₂ vents

marine ecology
an evolutionary perspective

<figure><img src="image_12.png" /></figure>

Marine Ecology. ISSN 0173-9565

### ORIGINAL ARTICLE

# Effects of ocean acidification and high temperatures on the bryozoan *Myriapora truncata* at natural CO₂ vents

Riccardo Rodolfo-Metalpa<fnref n="1" />,<fnref n="2" />*, Chiara Lombardi<fnref n="3" />*, Silvia Cocito<fnref n="3" />, Jason M. Hall-Spencer<fnref n="2" /> & Maria Cristina Gambi<fnref n="4" />

### Introduction to the study

Several papers started demonstrating that although OA decrease the calcification rates of ecologically important organisms such as coralline algae (Kuffner et al. 2008; Martin et al. 2008), foraminiferans (e.g. Moy et al. 2009), corals (e.g. Silverman et al. 2009), echinoderms (e.g. Michaelidis et al. 2005) and molluscs (Gazeau et al. 2007), with rates of calcification have been predicted to fall by up to 60% within this century, depending on the physiology of the species and their mineralogy (Kleypas et al. 2006). However, responses were highly species-specific (Ries et al. 2009) and sometimes differences and inconsistencies between studies were due, among other confusing factors (light, temperature, pH manipulation, seawater current velocity, foods, etc), to the different mineralogy of the organisms, and the skeleton exposition. Shells can dissolve when exposed to seawater with low carbonate saturation states such as in estuaries (Marshall et al. 2008), in upwelling areas (Feely et al. 2008) and around CO₂ vents (Hall-Spencer et al. 2008). Shells and/or skeletons made of high Mg-calcite are highly susceptible to dissolution as carbonate saturation states fall, followed by aragonitic skeletons and finally low Mg-calcite skeletons.

### The experiment

Here, we investigated rates of calcification and dissolution of the robust, branched bryozoan *M. truncata* (Pallas 1766).

Although 15% of the species of bryozoan are aragonitic and 17% are bimineralic (Smith et al. 2006), *M. truncata* is typical of most Bryozoa in that it has a calcitic skeleton. Our study aimed to investigate the effects of four-month in situ exposure to different pH conditions on the calcification and dissolution of *M. truncata* using in situ transplantation experiments at natural volcanic CO₂ vent sites. We test the hypothesis that temperature affects the degree to which ocean acidification affects net calcification and dissolution in this bryozoan. Live colonies and dead ones were transplanted to normal (pH 8.1), high (mean pH 7.66, minimum value 7.33) and extremely high-CO₂ conditions (mean pH 7.43, minimum value 6.83) at CO₂ vents off Ischia island (Tyrrhenian Sea, Italy). The net calcification rates of live colonies and the dissolution rates of dead colonies were estimated by weighing after 45 days (May-June 2008) and after 128 days (July-October) to examine the hypothesis that high CO₂ levels affect bryozoan growth and survival differently during moderate and warm water conditions.

<footnote n="1">1 International Atomic Energy Agency, Marine Environment Laboratories, Principality of, Monaco</footnote>

<footnote n="2">2 Marine Institute, Marine Biology and Ecology Research Centre, University of Plymouth, Plymouth, UK</footnote>

<footnote n="3">3 ENEA Marine Environment Research Centre, Santa Teresa, La Spezia, Italy</footnote>

<footnote n="4">4 Stazione Zoologica Anton Dohrm, Naples, Laboratory of Functional and Evolutionary Biology, Benthic Ecology Group, Ischia, Italy</footnote>
## Main results and discussion

In the first observation period seawater temperatures ranged from 19°C to 24°C; dead *M. truncata* colonies dissolved at high-CO₂ levels (pH 7.66) whereas live specimens maintained the same net calcification rate as those growing at normal pH. In extremely high-CO₂ conditions (mean pH 7.43), the live bryozoans calcified significantly less than those at normal pH. Therefore, established colonies of *M. truncata* seem well able to withstand the levels of ocean acidification predicted in the next two hundred years, possibly because the soft tissues protect the skeleton from an external decrease in pH. However, during the second period of observation, a prolonged period of high seawater temperatures (25-28°C) halted calcification both in controls and at high CO₂ and all transplants died when high temperatures were combined with extremely high-CO₂ levels.

<figure><img src="image_13.png" /></figure>

**Fig. 4.4.1.** Net calcification (a) measured during Period 1 and Period 2, and dissolution rates (b) measured during Period 1 respectively on live and dead transplanted colonies of *M. truncata* at extremely low (B1), low (B2) and normal pH (C) near and outside CO₂ vents at Ischia. Data are mean ± s.e. (n = 8 and 5). The mean and minimum pH measured during the whole experiment are reported in brackets.

Regarding the ability to calcify even under lo (B2) and extreme pH levels (B1), the results were not surprising since we expected to find no effect at pH 7.66 and a low growth rate at pH 7.43. However, from our data, it was not clear if this decrease was due to either the loss in the ability to calcify or the increase in the dissolution of the skeleton. Indeed, the buoyant weight technique (Davies et al. 1994) here used is the sum of both and measures the net calcification. When this work was performed, the measurement of gross calcification was difficult to perform because it needed the use of radioactive calcium. So, in this study, the gross calcification was calculated for the B2 samples by adding CaCO₃ dissolution to the net calcification rates measured on dead and live fragments. We found that the calcification was three times higher than the net calcification rates measured under normal conditions. An increase in calcification under acidified conditions has recently been reported for several species (Wood et al. 2008; Findlay et al. 2009; Ries et al. 2009). However, before firm conclusions are made about the ability of *M. truncata* to increase its calcification rate under high CO₂ conditions, more experiments are necessary using accurate calcification methods such as <sup>45</sup>Ca uptake (Tambutté et al. 1995).
With regard to skeleton dissolution, during this initial 45-day period, dead *M. truncata* skeletons did not dissolve and were heavily colonized by epibionts at normal pH, therefore increasing their weight, while they were dissolving both in B2 (90 mg g⁻¹ 30 day⁻¹) and at a very high rate in B1 (344 mg g⁻¹ 30 day⁻¹). Dissolution at B1 and B2 occurred even though these treatments normally experienced saturated Ωcalcite levels. This is likely due to both periods of carbonate undersaturation which occur at the site when the sea state is particularly calm (Hall-Spencer *et al.* 2008) and during the night.

Concerning colony survival, although adult *M. truncata* colonies were resilient to acidified conditions in the cooler part of our study (Period 1), at the end of Period 2, all B1 specimens had died, while fragments in B2 and C, although still living, showed near-zero calcification rates. Indeed, from 16 May 2008 to 26 June 2008 (Period 1) the water around the transplants warmed steadily from 19 to 24°C and the bryozoans grew well; then, during Period 2, it remained abnormally high at 25-28°C for three months. The mortality of all samples maintained 128 days under severe hypercapnia was presumably due to the synergistic effect of elevated seawater temperature and prolonged exposure to low pH levels. Although *M. truncata* was resilient to short-term exposure to high levels of ocean acidification at normal temperatures, our field transplants showed that its ability to calcify at higher temperatures was compromised, adding it to the growing list of species now threatened by global warming.

<figure><img src="image_14.png" /><figcaption>**Fig. 4.4.2.** *Myriapora truncata* maintained 128 days at mean pH 7.43 in B1 (a, b) showing the breakage of the zooidal soft tissues and the dissolution of the skeleton; dead fragments after 45 days at mean pH of 8.06, and 7.43 (d) in C and B1, respectively.</figcaption></figure>
## 4.5. Coral and mollusc resistance to ocean acidification adversely affected by warming

### LETTERS

PUBLISHED ONLINE: 21 AUGUST 2011 | DOI: 10.1038/NCLIMATE1200

nature
climate change

# Coral and mollusc resistance to ocean acidification adversely affected by warming

R. Rodolfo-Metalpa¹,²*, F. Houlbrèque¹†, É. Tambutté³, F. Boisson¹, C. Baggini², F. P. Patti⁴, R. Jeffree¹†,
M. Fine⁵,⁶, A. Foggo², J-P. Gattuso⁷,⁸ and J. M. Hall-Spencer²

This study has been possible thanks to the collaboration with E. Tambutté (Centre Scientifique de Monaco) and Fanny Houlbreque, who helped me with the ⁴⁵Ca technique I performed at the IAEA centre (Monaco), and again to E Tambutté for all the SEM he performed. The study was done in the framework of the project funded by the Prince Albert Foundation (F Boisson PI). This is my best paper!

Here, we show that corals and molluscs transplanted along gradients of carbonate saturation state at Mediterranean CO₂ vents can calcify and grow at even faster than normal rates when exposed to the high CO₂ levels projected for the next 300 years. Calcifiers remain at risk, however, due to the dissolution of exposed shells and skeletons that occurs as pH levels fall. Our results show that tissues and external organic layers play a major role in protecting shells and skeletons from corrosive seawater, limiting dissolution and allowing organisms to calcify. Our combined field and laboratory results demonstrate that the adverse effects of global warming are exacerbated when high temperatures coincide with acidification.

## Introduction to the study

The impact of acidification on the ability of individual species to calcify has remained elusive, however, as measuring net calcification (NC) fails to disentangle the relative contributions of gross calcification (GC) and dissolution rates on growth. Previous workers have not determined the impact of acidification on the ability of individual species to calcify since NC was measured [i.e. (GC) minus dissolution] which fails to disentangle the relative contributions of GC and dissolution rates. A decrease in NC could result from a decrease in GC, an increase in dissolution rates, or both. Recent studies indicate inconsistencies in the use of carbonate saturation state to predict marine calcification since some species can maintain, or even increase, their NC under low pH conditions. It is thought that this species-specific sensitivity is due to the presence and composition of external organic layers, and an ability to elevate pH and [CO₃²⁻] at sites of calcification. For example, measurements of the boron isotopic composition in *Cladocora caespitosa* held in aquaria at pHₜ 7.8 and from CO₂ vents showed that this Mediterranean coral was able to maintain a higher internal pH and hence ΔpH relative to ambient seawater pH17, therefore allowing calcification in undersaturated seawater. Calcification is known to occur between the tissue and the shell or skeleton where extrapallial fluid (in molluscs) and extracellular calcifying fluid (in corals) pH is 0.5 to > 1 unit higher than in ambient seawater.

## The experiment

We used a series of transplantation experiments along natural pCO₂ gradients at volcanic vents off Ischia (Tyrrhenian Sea, Italy) to investigate whether the projected reduction in calcification in key marine habitats by 2100 and beyond is a result of the suppression of GC, carbonate dissolution or both. We compared (1) a mollusc that has a periostracum covering the outside of
the shell (*Mytilus galloprovincialis*) with one that lacks a periostracum (*Patella caerulea*) and (2) the zooxanthellate coral *Balanophyllia europaea*, which has a skeleton completely covered in tissue with *Cladocora caespitosa* which has skeletal parts that are exposed to the surrounding seawater (Fig. 1). Shell and coral dissolution rates and coral NC rates were measured by buoyant weighing samples whilst GC rates of all four species were measured in aquaria with the radiotracer $^{45}$Ca after incubation *in situ*.

**Main results and discussion (on corals only)**

<figure><img src="image_15.png" /></figure>

**Fig. 4.5.1.** Calcification rates of corals transplanted along a CO₂ gradient off Ischia. **a**, net calcification (NC) measured after three months in 2008. Different symbols (for *C. caespitosa*) and letters (for *B. europaea*) are significantly different (*P* < 0.05). Data are means ± s.d. (n = 12 for *C. caespitosa*; n = 14 for *B. europaea*). **b**, gross calcification (GC) measured in aquaria with $^{45}$Ca labelled seawater at pHₜ 7.4, 7.7 and 8.1 on samples transplanted up to 7 months in 2009. *C. caespitosa* GC was only measured in June 2009 after three months. Data are means ± s.d. (n = 16 for *C. caespitosa*; n = 10 for *B. europaea*).

*Gross vs net calcification.* As expected from other coral studies, NC after three months decreased significantly with decreasing pH in our coral transplants (*C. caespitosa*: *F*<sub>3,44</sub> = 50; *P* < 0.001; *B. europaea*: *F*<sub>3,52</sub> = 17; *P* < 0.001). *C. caespitosa* had significantly slower linear growth rates as pH decreased (*F*<sub>2,65</sub> = 62; *P* < 0.001; mean ± s.d.: 0.19 ± 0.03, 0.32 ± 0.05, and 0.34 ± 0.07 mm month<sup>-1</sup>, at mean pH<sub>T</sub> 7.5, 7.8 and 8.1 respectively). At mean pH<sub>T</sub> 7.3 all the transplanted colonies had dissolved after *ca*. five months so no measurements were possible. In *C. caespitosa*, NC rates became negative at mean pH<sub>T</sub> < 7.5 (mean Ω<sub>a</sub> = 1.69, minimum Ω<sub>a</sub> = 0.40) but remained positive for *B. europaea* even at mean pH<sub>T</sub> 7.3 (mean Ω<sub>a</sub> = 1.13, minimum Ω<sub>a</sub> = 0.33).

Labelling with $^{45}$Ca showed that, remarkably, both coral species were able to maintain GC at pH levels projected for 2100 and beyond. Gross calcification in *C. caespitosa* was similar at pH<sub>T</sub> 8.1 and 7.7 and decreased by 30% at pH<sub>T</sub> 7.4 (*F*<sub>2,45</sub> = 10; *P* < 0.001), whereas GC in *B. europaea* significantly increased with decreasing pH in June and July (*F*<sub>2,54</sub> = 26; *P* < 0.001). This raises the possibility, as in other corals that *Balanophyllia* may rely on HCO₃⁻ for calcification.
<figure><img src="image_16.png" /></figure>

**Fig. 4.5.2.** Underwater and SEM images of *Balanophyllia europaea* transplanted along a CO₂ gradient off Ischia. **A-c**, live coral after seven months at mean pH_T 8.1 and (**d-f**) at mean pH_T 7.3. **g-i**, dead coral after three months at mean pH_T 7.3. Details of the outer corallite wall showing normal skeleton when covered (Co) in tissue (**b**, **e**) and (**h**) dissolved skeleton when uncovered (Un) in tissue. Enlargements (yellow boxes on **b**, **e**, **h**) show organized (**c**, **f**) and dissolved (**i**) bundles of aragonite crystals. Scale bars = 1 cm (a, d, g), 100 µm (b, e, h) and 1 µm (c, f, i).

***Skeleton dissolution.*** The dissolution of live samples transplanted at the vents was species-specific. While *C. caespitosa*, which has large parts of its skeleton exposed, showed evident marks of dissolution, *B. europaea* has a skeleton that is completely covered in tissue and was unaffected. The morphology of the exposed skeletons of live corals, as well as dead samples of both corals, maintained at mean pH_T 7.3 for three months, showed disordered aragonite crystals. In contrast, the skeleton covered in tissue had organized bundles of fine aragonite crystals as in samples maintained at normal pH. Just as periostracum protected mussel shells, coral tissues protected the skeletons from corrosive (Ω_ara < 1) seawater. This protective role may explain why some corals increase tissue thickness when seawater pH is lowered.
<figure><img src="image_17.png" /></figure>

**Fig. 4.5.3.** Single corallites of *Cladocora caespitosa* transplanted along a CO₂ gradient. **A-c**, live corals after 7 months at mean pHₜ 8.1; after three (**d-f**) and 7 months (**g-i**) at mean pHₜ 7.3. **l-n**, dead samples after three months at mean pHₜ 7.3. Details of the external calyx skeleton organisation (**b, e, h, m**). Enlargements (yellow boxes on **b, e, h, n**) of aragonite crystals showing normal (**c, f**) and dissolved (**i, n**) aragonite. The polyp of this species only covers the apical part of each corallite, leaving the skeleton exposed to the surrounding seawater. Corals transplanted to mean pHₜ 7.3 (**g**) showed the exposed skeleton highly damaged and a gradual polyp

retraction into the calyx (as indicated by the arrow), likely due to the effect of acidified seawater on the border between tissue and skeleton. This response increased the surface of exposed coral skeleton (Un = uncovered), which eventually dissolved, while parts still covered in tissue (Co = covered) maintained normal skeletal morphology. Scale bars are 0.3 cm (a, d, g, l), 100 µm (b, f, h, m) and 10 µm (c, f, i, n).

<figure><img src="image_18.png" /></figure>

**Fig. 4.5.4.** *Balanophyllia europaea* transplanted along a CO₂ gradient. **A**, after three months at mean pHₜ 7.2 the polyp retracted, likely due to the interaction with algae which fixed on the resin, therefore exposing part of the external calyx wall during the incubation. **B**, whole corallite showing dissolved uncovered (Un) and normal skeleton (Co). **c-f**, enlargements (yellow boxes in b) of the external wall (**c, e**) and of the calyx rim (**d**) showing normal skeleton organisation when the

skeleton was covered in tissue (Co). **f**, enlargement of the external wall showing the dissolved skeleton. Scale bars are 0.5 cm (a, b), 100 µm (c, d) and 1 µm (e, f).
## 4.6. Calcification is not the Achilles' heel of cold-water corals in an acidifying ocean

<figure><img src="image_19.png" /></figure>

Global Change Biology

Global Change Biology (2015) 21, 2238-2248, doi: 10.1111/gcb.12867

# Calcification is not the Achilles' heel of cold-water corals in an acidifying ocean

RICCARDO RODOLFO-METALPA¹,², PAOLO MONTAGNA³, STEFANO ALIANI⁴, MIRENO BORGHINI⁴, SIMONEPIETRO CANESE⁵, JASON M. HALL-SPENCER⁶, ANDY FOGGO⁶, MARCO MILAZZO⁷, MARCO TAVIANI³,⁸ and FANNY HOULBRÈQUE¹,²

Before starting the PhD I participated in some cruises on board Italian oceanographic vessels, in collaboration with the ENEA and the CNR. Thanks to these collaborations, my position at the IAEA in Monaco, and the research activities made in Ischia, I had the unique opportunity to collect some CWC, measure in the IAEA lab their gross calcification with isotopic technique, and finally transplant them for months both at the CO₂ seep of Ischia and at 350 m depth on a mooring line. This experiment was the first of its kind and revealed that CWC calcification was not fundamentally affected by OA.

## Introduction to the study

Research into the effects of acidification on CWCs in aquaria has provided conflicting results. Form & Riebesell (2012) found a significant decrease in the net calcification of *Lophelia pertusa* during a one-week exposure to high CO₂ levels but found that this reef-forming coral was able to acclimate in the long-term (6 months), increasing its calcification rate in seawater that was undersaturated concerning aragonite compared to saturated conditions. Work on other CWCs revealed that their calcification was not affected by acidification during the first 182 and 240 days of incubation (Movilla et al., 2014a; Carreiro-Silva et al., 2014 respectively). On the other hand, *Dsmophyllum dianthus* had a 70% reduction in skeletal growth rate after 314 days whereas *Dendrophyllia cornigera* showed no differences between treatments (Movilla et al., 2014a). Other studies have shown that calcification rates of *L. pertusa* and *Madrepora oculata* (which also forms deep-water reefs) were unaffected by future projected pCO₂ levels in short-(hours-days; Maier et al., 2012) and long-term (weeks-months; Maier et al., 2013a,b) experiments. McCulloch et al. (2012b) showed that CWCs can calcify at or close to the aragonite saturation horizon by elevating their internal pH, thus buffering external changes in seawater pH. These studies imply that shoaling of the aragonite saturation horizon may not cause the dramatic declines in coral calcification rates that were first feared (Guinotte et al., 2006; Jackson et al., 2014). However, to what extent undersaturated seawater might affect reef integrity by increasing the dissolution of exposed coral skeletons has not yet been investigated and all studies showing that they maintain calcification rates at high pCO₂ levels have been carried out in aquaria where feeding may have artificially boosted their energy reserves, likely altering responses to acidification.

## The experiment

In this study, we first measured the effects of acidification on the net and gross calcification rates as well as the respiration rates of three CWC species cultured in aquaria under present and future Representative Concentration Pathway (RCP) 8.5 pCO₂ scenario (IPCC, 2014). In addition, we quantified skeletal dissolution rates of *D. dianthus* maintained in aquaria during two-month incubations at present and high pCO₂ levels. Finally, we compared the calcification rate of *D. dianthus* fed in aquaria in aragonite-saturated conditions with those of corals
transplanted into undersaturated (CO₂ seeps of Ischia) and saturated conditions (at 350 m depth) off Italy.

*Caryophyllia smithii, Desmophyllum dianthus* and *Dendrophyllia cornigera* were collected during three cruises in the Med Sea and transported to the IAEA laboratory where they were maintained in the dark at a seawater temperature of 13°C and fed twice per week with frozen krill or freshly hatched *Artemia nauplii*. Two sets of corals were acclimated for one month either to ambient pH and pCO₂ (pH<sub>T</sub> = 8.07; pCO₂ = 319 µatm), or at levels projected by the end of the century (pH<sub>T</sub> = 7.70; pCO₂ = 1058 µatm), and maintained for further two months. Their net and gross calcification rates were measured.

A set of the most abundant *Desmophyllum dianthus* was suspended at 350 m depth on an instrumented mooring cable in the Corsica channel. Corals were weighed and attached to individual plates, which were put into an open cage fixed to the mooring cable. In Ischia, they were transplanted during the coldest month onto a rocky seabed at CO₂ seep sites previously studied by Rodolfo-Metalpa *et al*. (2011): 1) station B1 at 3 m depth with a mean pH of 7.43 ± 0.31; 2) and station C at 5 m depth with a mean pH of 8.06 ± 0.07. At both sites corals were positioned calyx upwards inside an open cage made with two PVC plates (60 x 90 cm), mounted using bolts and attached to 30 kg concrete blocks. Corals were maintained for 258 days at 350 m depth and for 43 days at 3 m depth.

<figure><img src="image_20.png" /><figcaption>**Fig. 4.6.1.** Transplantations of *D. dianthus* suspended at 350 m depth on a mooring off Corsica.</figcaption></figure>

**Main results and discussion**

We found that gross and net calcification rates of *Desmophyllum dianthus*, *Caryophyllia smithii* and *Dendrophyllia cornigera*, as well as dissolution rates of exposed skeleton and respiration rates of living *D. dianthus*, did not significantly change when exposed to high seawater pCO₂ (pH<sub>T</sub> = 7.70; pCO₂ = 1058 µatm, Ω<sub>ara</sub> = 1.29). We tested this observation further by transplanting *D. dianthus* to 350 m depth at ambient seawater conditions (pH<sub>T</sub> = 8.02; pCO₂ = 448 µatm; Ω<sub>ara</sub> = 2.58) and into undersaturated seawater (pH<sub>T</sub> = 7.35; pCO₂ = 2879 µatm; Ω<sub>ara</sub> = 0.76) near CO₂ seeps and confirmed that net calcification rates were not affected by the differences in seawater carbonate chemistry. This is likely linked to their ability to maintain a high Ω<sub>ara</sub> level and pH at the calcification site. It is well known that to build their skeletons, corals pump protons out of the extracellular calcifying medium to increase internal pH and favour calcification. This is highly energy consuming (Allemand *et al.* 2011) with an estimated metabolic extra cost of ca. 10% per 0.1 pH unit decrease in seawater pH (McCulloch *et al.* 2012b). To meet this energy demand, corals can increase feeding rates and/or draw upon energy reserves. In our aquaria experiment, corals were fed twice a week, which is lower than several