# Microalgae-bacteria interactions in the picoeukaryote photosynthetic model: Ostreococcus tauri

Josselin Lupette, Sophie Sanchez-Brosseau

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## To cite this version:

Josselin Lupette, Sophie Sanchez-Brosseau. Microalgae-bacteria interactions in the picoeukaryote photosynthetic model: Ostreococcus tauri. 6ème journée André Picard, Apr 2015, Villefranche sur mer, France. hal-05350740

HAL Id: hal-05350740

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Submitted on 6 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Microalgae-bacteria interactions in the picoeukaryote photosynthetic model : *Ostreococcus tauri RCC4221*

**Josselin LUPETTE**<fnref n="1" />,<fnref n="2" />*, **Sophie SANCHEZ-FERANDIN**<fnref n="1" />,<fnref n="2" />

## Introduction

The "Phycosphere" - the close region surrounding and influencing the phytoplankton cells - is an important bacterial habitat (1). Some heterotrophic bacteria are known to enhance (2) or inhibit (3) the growth of microalgae. Three kinds of interactions have been currently described : the **free-living bacteria** (4) without direct contact, the **epiphytic bacteria** (5) adhering to the surface of microalgae and the **endophytic bacteria** (6) growing inside the microalgae.

Al : Algaecide / Atb : Antibiotics / IM : Inorganic Matter / N : NH₄⁺ / OM : Organic Matter / P : PO₄³⁻ / Ph : Phytohormones / Pho : Photosynthesis / R : Respiration / Sid : Siderophore.

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We are interested, here, in the study of microalgae-bacteria interactions in the model *Ostreococcus tauri RCC4221* (Chlorophyta, Mamiellophyceae). Discovered in the Thau lagoon (43°24′N, 03°36′E) (7), this microalgae (0,8µm) has a **minimal cellular organization** with a haploid nuclear genome of 12,6Mpb distributed over 20 chromosomes.

The presence of bacteria is often detected in cultures of *Ostreococcus tauri* in the laboratory. However, the types of interactions between these actors are still unknown.

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

## Objectives

- Estimate the **1) proportion** and the **2) diversity of bacteria** during the growth of *Ostreococcus tauri* RCC4221 in different media and different photoperiods.

- Optimize an **3) axenisation protocol** of this microalgae to begin experiments in the presence of bacterial strains isolated in the laboratory.

### 1) Abundance of bacteria

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Flow cytometer analysis
SYBR Green-I staining for bacteria
detection</figcaption></figure>

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### 2) Diversity of cultivable bacteria

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

n=48 clones per time / current sequencing: 20 to 35 days

### 3) Axenisation protocol

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

**Conventional lab's protocol**

Vancomycin : 100 mg mL⁻<fnref n="1" />
Ciprofloxacin : 10 mg mL⁻<fnref n="1" />

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

<figure><img src="image_14.png" /><figcaption>Flow
Cytometry
analysis</figcaption></figure>

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

<figure><img src="image_16.png" /><figcaption>[Atb]</figcaption></figure>

<figure><img src="image_17.png" /></figure>

C : Centrifugation
D_c : Dunaliella
Antibiotics Cocktail
V : Vancomycin
Ci : Ciprofloxacin
D : Dilution

## Conclusion

1) **Strong presence of bacteria** in the cultures of *Ostreococcus tauri* RCC4421

2) *Marinobacter sp.* is the bacteria genus the most represented in the **bacterial cultivable fraction** whatever the **culture medium** used or the **applied photoperiod**.

3) **High effect of centrifugation** on the elimination of the bacterial population. The *Dunaliella sp.* antibiotic cocktail protocol can maintain low percentage of bacteria in the L1-MOLA environment. The routine laboratory protocol seems to have **no effect** on bacterial populations of *Ostreococcus tauri* cultures.

## Prospects

1) Construct a 16S rDNA consensus phylogenetic tree.

2) Observe these cultures by **electron microscopy** to determine the kind of interactions among microalgae and bacteria.

3) Continue the **axenisation work** of microalgae cultures.

4) Test other environmental parameters (T, pH, salinity, nutrient concentration, ...).

5) Realize a **high-throughput sequencing** (Illumina HiSeq) to measure the whole bacterial diversity.
→ Develop the same scientific approach among other Mamiellophyceae species (*Bathycoccus sp.*, *Micromonas sp.*).

## References

(1) : Bell, W., Mitchell, R. 1972. « Chemotactic and growth responses of marine bacteria to algal extracellular products ». *Biol Bull.* **143**: 265-277.

(2) : Liu, J., Lewitus, A.J., Brown, P., Wilde, S.B. 2008. « Growth-promoting effects of a bacterium on raphidophytes and other phytoplankton » *Harmful Algae*. **7**: 1-10.

(3) : Meseck, S.L., Smith, B.C., Wikfors, G.H., Alix, J.H., Kapareiko, D. 2007. « Nutrient interactions between phytoplankton and bacterioplankton under different carbon dioxide regimes ». *Journal of Applied Phycology*. **19**: 229-237.

(4) : Le Chevanton, M. 2013. Interactions microalgues-bactéries en système expérimental bispécifique : effets sur la croissance de *Dunaliella sp.* 198 pages. Thèse de phycologie, Université de Nantes - Faculté des Sciences et des Techniques.

(5) : Bates, S.S., Gaudet, J., Kaczmarska, I., Ehrman, J.M. 2004. « Interaction between bacteria and the domoic-acid-producing diatom *Pseudo-nitzschia multiseries* (Hasle) Hasle; can bacteria produce domoic acid autonomously? » *Harmful Algae* **3**:11-20

(6) : Kawafune, K., Hongoh, Y., Hamaji, T., Nozaki, H. 2012. « Molecular identification of rickettsial endosymbionts in the non-phagotrophic Volvocalean green algae ». *PLoS ONE* **7**:e31749.

(7) : Courties, C., Vaquer, A., Troussellier, M., Lautier, J., Chrétiennot-Dinet, M.J., Neveux, J., Machado, M.C., Claustre, H. 1994. « Smallest eukaryotic organism ». *Nature*. **370**: 255.

## Abbreviations

BLASTn : nucleotidic Basic Local Alignment Search Tool

ESAW : Enriched Seawater, Artificial SeaWater

FL1 : Green Fluorescence

HNA : High Nucleic Acid

LNA : Low Nucleic Acid

MOLA : Microbial Observatory Laboratory Arago

PCR : Polymerase Chain Reaction

RCC : Roscoff Culture Collection

RT : Room Temperature

SSC : Side Scatter

<footnote n="1">¹Team Evolutionary and Environmental Genomics of Phytoplankton - CNRS, UMR 7232 - Integrative Biology of Marine Organisms, Observatoire Océanologique, Banyuls sur Mer, France</footnote>

<footnote n="2">²Observatoire Océanologique, Sorbonne Universités - Pierre and Marie Curie University, Banyuls sur Mer, France</footnote>

<footnote>* Correspondance : [lupettejosselin@yahoo.fr](mailto:lupettejosselin@yahoo.fr)</footnote>