Acute warming combined with hypoxia and hypercapnia challenges but does not overwhelm <i>Ostrea edulis</i> passive tolerance mechanisms

Hypoxia and hypercapnia often accompany seawater warming and interactively alter marine ectotherm performance, potentially threatening their populations. To detail mechanistic responses, we investigated whole-animal physiology alongside cellular homeostasis in a species expected to be relatively robust to their impacts, the oyster Ostrea edulis. Acute warming alone and combined with hypercapnia and hypoxia (deadly trio, DT) started at 18°C, increasing stepwise by 2°C per 48 h until critical temperature was reached (34°C). Death of oysters started at a lower temperature under DT than under acute warming alone, but rates equalized by 34°C. Hemolymph PO2 in DT-exposed oysters was 29% lower at 18°C, but by 34°C, was only slightly lower than that in oysters subjected to acute warming alone. In both groups, resting metabolic rate (RMR) and heart rate rose with warming. Hemolymph PO2 was stable until 26°C, whence it declined. DT elicited a higher heart rate, which began to fall after ∼32°C, whereas heart rate in oysters subjected to acute warming continued rising. Relative increases in branchial metabolite levels of alanine and fumarate, profiled via 1H-NMR spectroscopy, indicated greater contributions of anaerobic metabolism in DT-exposed oysters. Gill tissue showed higher levels of the mitochondrial stabilizer sirtuin-5 (SIRT5) alongside higher antioxidative capacity under DT compared with acute warming, before declining at temperatures beyond 30°C. Muscle intracellular pH, gill heat shock protein 70 and metabolic profiles appeared unaffected by DT compared with warming alone. Our results suggest that DT places an additional energetic burden on the oyster, lowering the critical temperature. Nevertheless, tolerance patterns indicate resilience to DT, which may require a rebalancing of passive tolerance mechanisms, especially metabolic depression.