Differential impacts of pH on growth, physiology, and elemental stoichiometry across three coccolithophore species

Coccolithophores are pivotal players in ocean biogeochemistry, yet the impact of changing pH on the physiology of different species remains unclear as there has been a dominant focus on Gephyrocapsa huxleyi. Meta-analyses of existing experimental data are challenging due to the differences in multidimensional 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 possibly due to high [H⁺]-related inhibition. Low pH impacted cellular physiology and elemental stoichiometry, while the impact of high pH was less adverse. The changes in elemental production induced by low pH could exert a negative influence on the contribution of coccolithophores to nutrient and carbon export, especially for biogeochemically relevant open-ocean species. pH also affected coccolith formation, especially in C. braarudii, through CO₂ limitation at high pH and low calcite saturation state at low pH. Contrasting species-specific pH sensitivities highlighted the potential for species like G. huxleyi to further outperform others like C. braarudii in an acidic ocean. Literature synthesis showed that coccolithophores show a broad CO₂ optimum, although growth rates and particulate inorganic carbon to particulate organic carbon ratios consistently declined with increasing CO₂. Strain-specific CO₂ optima partly contributed to the variability within responses of individual species, giving the misleading perception of a broad species-level CO₂ optimum. Strain-specific optima exist possibly due to their adaptation to carbonate chemistry conditions at the place of origin.