| Summary: | Global ocean oxygen (O<sub>2</sub>) content is decreasing as climate change drives declines in oxygen solubility, strengthened stratification of seawater masses, increased biological oxygen consumption and coastal eutrophication. Studies on the biological effects of nocturnal decreased oxygen concentrations (hypoxia) on coral reefs are very scarce. Coral reefs are fundamental for supporting one quarter of all marine species and essential for around 275 million people worldwide. This study investigates acute physiological and photobiological responses of a scleractinian coral (<i>Acropora</i> spp.) to overnight hypoxic conditions (<2 mg/L of O<sub>2</sub>). Bleaching was not detected, and visual and physical aspects of corals remained unchanged under hypoxic conditions. Most photobiological-related parameters also did not show significant changes between treatments. In addition to this, no significant differences between treatments were observed in the pigment composition. However, hypoxic conditions induced a significant decrease in coral de-epoxidation state of the xanthophyll cycle pigments and increase in DNA damage. Although the present findings suggest that <i>Acropora</i> spp. is resilient to some extent to short-term daily oxygen oscillations, long-term exposure to hypoxia, as predicted to occur with climate change, may still have deleterious effects on corals.
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