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Exposure duration modulates the response of Caribbean corals to global change stressors

Global change is threatening coral reefs, with rising temperatures leading to repeat bleaching events (dysbiosis of coral hosts and their symbiotic algae) and ocean acidification reducing net coral calcification. Although global-scale mass bleaching events are revealing fine-scale patterns of coral resistance and resilience, traits that lead to persistence under environmental stress remain elusive. Here, we conducted a 95-day controlled-laboratory experiment to investigate how duration of exposure to ocean warming (28, 31°C), acidification (pCO2 = 400–2800 μatm), and their interaction influence the physiological responses of two Caribbean reef-building coral species (Siderastrea siderea, Pseudodiploria strigosa) from two reef zones of the Belize Mesoamerican Barrier Reef System. Every 30 days, calcification rate, total host protein and carbohydrate, chlorophyll a pigment concentration, and symbiont cell density were quantified for the same coral colony to characterize acclimatory responses of each genotype. Physiologies of the two species were differentially affected by these stressors, with exposure duration modulating responses. Siderastrea siderea was most affected by extreme pCO2 (~2800 μatm), which resulted in reduced calcification rate, symbiont density, and chlorophyll a concentration. Siderastrea siderea calcification rate initially declined under extreme pCO2 but recovered by the final time point, and overall demonstrated resistance to next-century pCO2 and temperature stress. In contrast, P. strigosa was more negatively impacted by elevated temperature (31°C). Reductions in P. strigosa calcification rate and total carbohydrates were consistently observed over time regardless of pCO2 treatment, with the greatest reductions observed under elevated temperature. However, nearshore colonies of P. strigosa maintained calcification rates under elevated temperature throughout all exposure durations, suggesting individuals from this environment may be locally adapted to the warmer temperatures characterizing their natal reef zone. This experiment highlights how tracking individual coral colony physiology across broad exposure durations can capture acclimatory responses of corals to global change stressors.