A physicochemical framework for interpreting the biological calcification response to CO2-induced ocean acidification

A generalized physicochemical model of the response of marine organisms’ calcifying fluids to CO₂-induced ocean acidification is proposed. The model is based upon the hypothesis that some marine calcifiers induce calcification by elevating pH, and thus Ω_A, of their calcifying fluid by removing protons (H⁺). The model is explored through two end-member scenarios: one in which a fixed number of H⁺ is removed from the calcifying fluid, regardless of atmospheric pCO₂, and another in which a fixed external-internal H⁺ ratio (H⁺]_E/H⁺]_I) is maintained. The model is able to generate the full range of calcification response patterns observed in prior ocean acidification experiments and is consistent with the assertion that organisms’ calcification response to ocean acidification is more negative for marine calcifiers that exert weaker control over their calcifying fluid pH. The model is empirically evaluated for the temperate scleractinian coral Astrangia poculata with in situ pH microelectrode measurements of the coral’s calcifying fluid under control and acidified conditions. These measurements reveal that (1) the pH of the coral’s calcifying fluid is substantially elevated relative to its external seawater under both control and acidified conditions, (2) the coral’s H⁺]_E/H⁺]_I is approximately the same under control and acidified conditions, and (3) the coral removes fewer H⁺ from its calcifying fluid under acidified conditions than under control conditions. Thus, the carbonate system dynamics of A. poculata’s calcifying fluid appear to be most consistent with the fixed H⁺]_E/H⁺]_I end-member scenario. Similar microelectrode experiments performed on additional taxa are required to assess the model’s general applicability.