Coral pH regulation of the calcifying fluid is modulated by seawater dissolved inorganic carbon concentration

Reef coral calcification depends on regulation of pH in the internal calcifying fluid in which the coral skeleton forms. However, little is known about calcifying fluid pH (pHCF) regulation, despite its importance in determining the response of corals to ocean acidification. Here, we investigate the impact of seawater dissolved inorganic carbon (DIC) concentration on calcifying fluid pH in the coral Stylophora pistillata in seawater with manipulated [DIC] and constant pH. Our results reveal that regulation of pHCF and calcification rates are sensitive to changes in seawater [DIC] in the light and dark. While part of this relationship can be explained by changes in rates of photosynthesis and respiration, our data point to the importance of seawater DIC in pH regulation of the coral's calcifying cells. Our findings contribute towards a mechanistic understanding of how and why coral calcification is sensitive to changes in seawater carbonate chemistry, which is needed for predicting effects of environmental change on coral reefs and for robust interpretations of isotopic paleoenvironmental records in coral skeletons.