H+-driven increase in CO2 uptake and decrease in HCO3- uptake explain coccolithophores' acclimation responses to ocean acidification

Recent ocean acidification (OA) studies revealed that seawater [H] rather than [CO2] or [HCO 2 3 ] regulate short-term responses in carbon fluxes of Emiliania huxleyi. Here, we investigated whether acclimation to altered carbonate chemistry modulates this regulation pattern and how the carbon supply for calcification is affected by carbonate chemistry. We acclimated E. huxleyi to present-day (ambient [CO2], [HCO 2 3 ], and pH) and OA conditions (high [CO2], ambient [HCO 2 3 ], low pH). To differentiate between the CO2 and pH/H 1 effects, we also acclimated cells to carbonation (high [CO2] and [HCO 2 3 ], ambient pH) and acidification (ambient [CO2], low [HCO3 ], and pH). Under these conditions, growth, production of particulate inorganic and organic carbon, as well as carbon and oxygen fluxes were measured. Under carbonation, photosynthesis and calcification were stimulated due to additional HCO3 uptake, whereas growth was unaffected. Such stimulatory effects are not apparent after short-term carbonation, indicating that cells adjusted their carbon acquisition during acclimation. Being driven by [HCO3 ], these regulations can, however, not explain typical OA effects. Under acidification and OA, photosynthesis stayed constant, whereas calcification and growth decreased. Similar to the short-term responses toward high [H], CO2 uptake significantly increased, but HCO 2 3 uptake decreased. This antagonistic regulation in CO2 and HCO 2 3 uptake can explain why photosynthesis, being able to use CO2 and HCO3 , often benefits from OA, whereas calcification, being mostly dependent on HCO 2 3 , often decreases. We identified H as prime driver of coccolithophores’ acclimation responses toward OA. Acidified conditions seem to put metabolic burdens on the cells that result in decreased growth. Emiliania huxleyi is the Earth’s most dominant pelagic calcifier and known to be well adapted to shallow mixed-layer depths with high irradiances (Nanninga and Tyrrell 1996; Raitsos et al. 2006). Under these conditions, the species is able to form large monospecific blooms with cell concentrations of up to 10 3 10 cells L (Holligan et al. 1993; Tyrrell and Merico 2004). In the process of calcification, CO 3 precipitates intracellularly with Ca to form CaCO3, leading to reduced seawater CO 3 levels and alkalinity. This production of particulate inorganic carbon (PIC) furthermore increases the partial pressure of carbon dioxide (pCO2) of seawater and thereby counteracts the effect of photosynthetic production of particulate organic carbon (POC). The relative strength of calcification vs. photosynthesis therefore influences the biogeochemical CO2 fluxes on regional and global scales (Broecker and Peng 1987; Rost and Riebesell 2004). In the last decades, E. huxleyi has become an important model organism, especially because of its high sensitivity toward ocean acidification (OA; Raven and Crawfurd 2012; Read et al. 2013; Meyer and Riebesell 2015). This term describes the strong increase in CO2 and the slight increase in HCO3 levels (their sum is referred to as carbonation) as well as the decrease in CO 3 levels and pH (the latter corresponds to an increase in [H] and is referred to as acidification), which result from the oceanic uptake of anthropogenic CO2 (Wolf-Gladrow et al. 1999; Caldeira and Wickett 2003). A large number of laboratory and field studies on E. huxleyi and other coccolithophores found that OA leads to unaffected or stimulated photosynthesis, with impaired or unaffected calcification and growth, typically leading to decreased PIC: POC ratios (Raven and Crawfurd 2012; Kroeker et al. 2013; Meyer and Riebesell 2015). These responses can yet vary in magnitude, depending on genetic predisposition and other environmental boundary conditions such as light, temperature, or nutrient status (Zondervan *Correspondence: Dorothee.Kottmeier@awi.de This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 2045 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 61, 2016, 2045–2057 VC 2016 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10352