Physicochemical Control of Caribbean Coral Calcification Linked to Host and Symbiont Responses to Varying <i>p</i>CO₂ and Temperature

It is thought that the active physiological regulation of the chemistry of a parent fluid is an important process in the biomineralization of scleractinian corals. Biological regulation of calcification fluid pH (pH_CF) and other carbonate chemistry parameters ([CO₃²⁻]_CF, DIC_CF, and Ω_CF) may be challenged by CO₂ driven acidification and temperature. Here, we examine the combined influence of changing temperature and CO₂ on calcifying fluid regulation in four common Caribbean coral species—<i>Porites astreoides</i>, <i>Pseudodiploria strigosa</i>, <i>Undaria tenuifolia</i>, and <i>Siderastrea siderea</i>. We utilize skeletal boron geochemistry (B/Ca and δ¹¹B) to probe the pH_CF, [CO₃²⁻]_CF, and DIC_CF regulation in these corals, and δ¹³C to track changes in the sources of carbon for calcification. Temperature was found to not influence pH_CF regulation across all <i>p</i>CO₂ treatments in these corals, in contrast to recent studies on Indo-Pacific pocilloporid corals. We find that [DIC]_CF is significantly lower at higher temperatures in all the corals, and that the higher temperature was associated with depletion of host energy reserves, suggesting [DIC]_CF reductions may result from reduced input of respired CO₂ to the DIC pool for calcification. In addition, δ¹³C data suggest that under high temperature and CO₂ conditions, algal symbiont photosynthesis continues to influence the calcification pool and is associated with low [DIC]_CF in <i>P. strigosa</i> and <i>P. astreoides</i>. In <i>P. astreoides</i> this effect is also associated with an increase in chlorophyll a concentration in coral tissues at higher temperatures. These observations collectively support the assertion that physicochemical control over coral calcifying fluid chemistry is coupled to host and symbiont physiological responses to environmental change, and reveals interspecific differences in the extent and nature of this coupling.