Physiological and elemental stoichiometry changes in three coccolithophore species in response to changing seawater pH

Coccolithophores are pivotal players in ocean biogeochemistry, yet the impact of changing carbonate chemistry on the physiology of different species remains unclear as there has been a dominant focus on G. huxleyi. Meta-analyses of existing experimental data are challenging due to differences in the multi-dimensional culture conditions. This study investigated the response of three species – Gephyrocapsa huxleyi, Coccolithus braarudii and Chrysotila carterae – under varying CO2 conditions (via pH). The sensitivity to pH differed between species, but all species showed reduced growth rates under the highest CO2 (lowest pH) treatment due to H+-related inhibition. Changes in seawater pH impacted productivity and cell biochemistry, causing changes in cellular elemental stoichiometry. pH affected coccolith formation through CO2 limitation at high pH and low calcite saturation state at low pH. Literature synthesis showed that coccolithophores show a broad CO2 optimum, although growth rates and PIC:POC declined with increasing CO2. However, strain-specific CO2 optima contributed to variability within the responses of a single species, possibly due to adaptation to carbonate chemistry conditions at the site of isolation. Overall, changes in elemental stoichiometry could exert an influence on nutrient and carbon export. Higher CO2 levels (lower pH) are likely to cause reduced carbon and nutrient export from open-ocean coccolithophores, causing reduced food quality. Species-specific sensitivities to pH will likely allow species like G. huxleyi to outperform others like C. braarudii in a future acidified ocean. Strains with a growth optimum at higher CO2 levels may outcompete those with a growth optimum at lower CO2 levels.