Coral calcification in a changing World and the interactive dynamics of pH and DIC upregulation

Coral calcification is dependent on the mutualistic partnership between endosymbiotic zooxanthellae and the coral host. Here, using newly developed geochemical proxies (δ¹¹B and B/Ca), we show that Porites corals from natural reef environments exhibit a close (r² ∼0.9) antithetic relationship between dissolved inorganic carbon (DIC) and pH of the corals' calcifying fluid (cf). The highest DIC_cf (∼ × 3.2 seawater) is found during summer, consistent with thermal/light enhancement of metabolically (zooxanthellae) derived carbon, while the highest pH_cf (∼8.5) occurs in winter during periods of low DIC_cf (∼ × 2 seawater). These opposing changes in DIC_cf and pH_cf are shown to maintain oversaturated but stable levels of carbonate saturation (Ω_cf ∼ × 5 seawater), the key parameter controlling coral calcification. These findings are in marked contrast to artificial experiments and show that pH_cf upregulation occurs largely independent of changes in seawater carbonate chemistry, and hence ocean acidification, but is highly vulnerable to thermally induced stress from global warming.