A coastal coccolithophore maintains pH homeostasis and switches carbon sources in response to ocean acidification

Ocean acidification will potentially inhibit calcification by marine organisms; however, the response of the most prolific ocean calcifiers, coccolithophores, to this perturbation remains under characterized. Here we report novel chemical constraints on the response of the widespread coccolithophore species Ochrosphaera neapolitana (O. neapolitana) to changing-CO₂ conditions. We cultured this algae under three pCO₂-controlled seawater pH conditions (8.05, 8.22, and 8.33). Boron isotopes within the algae's extracellular calcite plates show that this species maintains a constant pH at the calcification site, regardless of CO₂-induced changes in pH of the surrounding seawater. Carbon and oxygen isotopes in the algae's calcite plates and carbon isotopes in the algae's organic matter suggest that O. neapolitana utilize carbon from a single internal dissolved inorganic carbon (DIC) pool for both calcification and photosynthesis, and that a greater proportion of dissolved CO₂ relative to HCO₃^- enters the internal DIC pool under acidified conditions. These two observations may explain how O. neapolitana continues calcifying and photosynthesizing at a constant rate under different atmospheric-pCO₂ conditions.