Mechanisms of coral pH regulation and calcification response to ocean acidification: Insights from a physicochemical model

Ocean acidification, induced by anthropogenic CO₂ emission, poses challenges for coral calcification and thus the health of coral reef ecosystems. The calcification responses of corals to this environmental change however appear to vary significantly among different species and reef systems, and are believed to be controlled by their up-regulation of the calcifying fluid pH. Here I present a physiochemical model of coral calcification to simulate the chemistry, especially pH, of the coral calcifying fluid and evaluate the coral calcification response to changing seawater chemistry. I show that, the extent of pH up-regulation in corals is controlled by the interplay between the enzymatic alkalinity pumping at the site of calcification (P) and the exchange of the calcifying fluid with external seawater (E). Corals with higher P/E ratios are predicted to elevate the pH of their calcifying fluid to higher extents, and thus should have higher calcifying fluid pH when exposed to the same seawater. Similarly, higher P/E ratios are also predicted to lead to higher saturation states of the calcifying fluid and therefore higher rates of coral calcification. This model quantitatively explains the variations of calcifying fluid pH and calcification responses observed among different coral species and laboratory manipulation experiments, and potentially enables predictions of coral calcification responses to future environmental changes.