Sublethal changes to coral metabolism in response to deoxygenation.

Coastal deoxygenation poses a critical threat to tropical coral reefs. Dissolved oxygen (DO) depletion can cause hypoxia-induced stress and mortality in scleractinian corals. Coral hypoxic responses are species-specific and likely modulated by the duration and severity of low-DO conditions, although the physiological mechanisms driving hypoxia tolerance are not fully understood. In this study, the Caribbean corals Acropora cervicornis, Porites astreoides and Siderastrea siderea were exposed to either severe (1.5 mg l-1 DO) or moderate (3.5 mg l-1 DO) deoxygenation or a control treatment (6 mg l-1 DO). All corals survived 2 weeks of deoxygenation but exhibited sublethal changes to coral metabolism after 1- and 2-week exposures, compared with controls. Maximum quantum yield (Fv/Fm) was suppressed after 1 week in both deoxygenation treatments in A. cervicornis, and after 2 weeks in S. siderea and P. astreoides exposed to severe or moderate treatments, respectively. Respiration rates were lower than controls in A. cervicornis and S. siderea after 1 and 2 weeks of severe deoxygenation. The reduced respiration of P. astreoides after 1 week of moderate deoxygenation returned to control levels in week 2. Overall coral metabolic budgets, assessed by ratios of gross photosynthesis to respiration (Pg:R), were more autotrophic, or photosynthesis-dominant, after 1 week of severe deoxygenation in S. siderea and P. astreoides, whereas Pg:R was not significantly different in A. cervicornis between treatments. These results reveal that some corals shift their metabolism to tolerate low-oxygen conditions and avoid bleaching or mortality, indicating that metabolic plasticity is an important aspect of coral resistance to deoxygenation.