Resistance of a reef-building coral to hypoxia

Coastal hypoxia has long been recognized as an important stressor in temperate marine habitats. Emerging research indicates that nearshore deoxygenation has equally catastrophic consequences in tropical marine systems. Some coral reefs may be exposed to persistent or acute hypoxic conditions over a range of spatial and temporal scales, which may select for increased resistance to hypoxic stress in some coral species. Yet, there remains a substantial gap in our understanding of the effects of deoxygenation in reef-building corals. To address this gap we tested the effects of exposure to a range of dissolved oxygen (DO) concentrations (0.5-6.0 mg L^-1) on the reef-building coral Siderastrea radians. We monitored coral photophysiology every 6 hours in response to DO treatments for 10 days. Sampling throughout the diel cycle allowed us to explore the potential for endosymbiont photosynthesis to influence coral responses. The most extreme DO treatments significantly depressed coral photophysiology, an indicator of coral physiological stress. However, the magnitude of response was influenced by the cumulative time of exposure to hypoxic conditions and sampling period in the diel cycle. The strongest effect of deoxygenation was just before sunrise, and became more apparent with longer exposure to treatment conditions. These findings indicate that some coral species may be resistant to extreme deoxygenation, and that their response is dependent on duration of exposure and time of day. The difference in day versus night responses suggests that photosynthetic production of oxygen may mediate hypoxia in coral tissues, and could be a key factor regulating the magnitude of negative effects of hypoxia on coral physiology. Our results shed light on the potential effects of ocean deoxygenation on a reef-building coral species, and provide insight into the capacity for coral resistance in a rapidly changing ocean climate.