Corals are globally important calcifiers that exhibit complex responses to anthropogenic warming and acidification. Although coral calcification is supported by high seawater pH, photosynthesis by the algal symbionts of zooxanthellate corals can be promoted by elevated pCO₂. To investigate the mechanisms underlying corals’ complex responses to global change, three species of tropical zooxanthellate corals (Stylophora pistillata, Pocillopora damicornis, and Seriatopora hystrix) and one species of asymbiotic cold-water coral (Desmophyllum pertusum, syn. Lophelia pertusa) were cultured under a range of ocean acidification and warming scenarios. Under control temperatures, all tropical species exhibited increased calcification rates in response to increasing pCO₂. However, the tropical species’ response to increasing pCO₂ flattened when they lost symbionts (i.e., bleached) under the high-temperature treatments—suggesting that the loss of symbionts neutralized the benefit of increased pCO₂ on calcification rate. Notably, the cold-water species that lacks symbionts exhibited a negative calcification response to increasing pCO₂, although this negative response was partially ameliorated under elevated temperature. All four species elevated their calcifying fluid pH relative to seawater pH under all pCO₂ treatments, and the magnitude of this offset (Δ[H⁺]) increased with increasing pCO₂. Furthermore, calcifying fluid pH decreased along with symbiont abundance under thermal stress for the one species in which calcifying fluid pH was measured under both temperature treatments. This observation suggests a mechanistic link between photosymbiont loss (‘bleaching’) and impairment of zooxanthellate corals’ ability to elevate calcifying fluid pH in support of calcification under heat stress. This study supports the assertion that thermally induced loss of photosymbionts impairs tropical zooxanthellate corals’ ability to cope with CO₂-induced ocean acidification.

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—Porites astreoides, Pseudodiploria strigosa, Undaria tenuifolia, and Siderastrea siderea. 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 pCO₂ 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 P. strigosa and P. astreoides. In P. astreoides 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.