Impacts of heat stress on the calcifying fluid chemistry and trace element composition of corals from thermally variable reefs

Coral reefs are increasingly threatened by climate change and mass bleaching events. Predicting how corals will respond to rapid ocean warming requires a better understanding of how they have responded to environmental change in the past - information that can be reconstructed from coral skeletal records. However, significant knowledge gaps remain in our understanding of how coral biomineralization and the incorporation of geochemical tracers is impacted by heat stress and bleaching, particularly since the physiological status of corals used for reconstruction of past stress events is often unknown. Using boron-based geochemical tracers (δ11B, B/Ca), we investigated how heat stress caused by a marine heatwave impacted the carbonate chemistry of the coral calcifying fluid as well as skeletal trace element composition in the branching coral Acropora aspera. Importantly, we recorded in situ temperature and coral health status during the bleaching event 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 temperature proxies Sr/Ca, Mg/Ca, Li/Ca and Li/Mg, likely due to a combination of reduced growth rates, disruption of key enzymes involved in calcification and Rayleigh fractionation. Anomalies in TE/Ca ratios were still observed 7 months after peak bleaching, even though symbiont densities and chlorophyll a concentrations were fully restored at this point. Interestingly, the response to heat stress did not differ between the thermally variable intertidal and the thermally more moderate subtidal environments whose coral populations are known to have a different heat tolerance, nor between colonies with varying degrees of bleaching. Our findings suggest that coral biomineralization mechanisms are highly sensitive to heat stress, and that the biogeochemical stress response of branching Acropora corals is remarkably consistent with that of massive Porites corals.