Heat stress differentially impacts key calcification mechanisms in reef-building corals

Coral reefs are increasingly threatened by climate change, mass bleaching events and ocean acidification (OA). Coral calcification, a process that is critical to build and maintain the structure of tropical coral reefs, is highly sensitive to both warming and acidifying oceans. However, in contrast to the impacts of OA on coral calcification, significant knowledge gaps remain regarding how coral biomineralization mechanisms are impacted by heat stress and bleaching. Using a combined physiological and geochemical approach, we investigated how a marine heatwave impacted coral symbiotic status (chlorophyll a, algal symbiont density), the carbonate chemistry of the coral calcifying fluid (via δ11B and B/Ca) and skeletal trace element composition in the branching coral Acropora aspera. Importantly, we recorded in situ temperature throughout the bleaching event and recovery as well as coral symbiotic status during peak bleaching and after 7 months of recovery. We show that heat-stressed Acropora corals continued to upregulate the pH of their calcifying fluid (cf); however, dissolved inorganic carbon upregulation inside the cf was significantly disrupted by heat stress. Similarly, we observed suppression of the typical seasonality in the trace element (TE) temperature proxies Sr/Ca, Mg/Ca, Li/Ca and Li/Mg, indicating disruption of important calcification mechanisms, Rayleigh fractionation and reduced growth rates. Anomalies in TE/Ca ratios were still observed 7 months after peak bleaching, even though algal symbiont densities and chlorophyll a concentrations were fully restored at this point. Interestingly, the biomineralization response to heat stress did not differ between thermally distinct reef habitats harbouring coral populations with different heat tolerance, nor between heat-stressed colonies with different severity of bleaching. Our findings suggest that coral biomineralization mechanisms in Acropora are highly sensitive to heat stress, showing similar patterns of biogeochemical stress response as other coral taxa.