Effects of Hypoxia on Coral Photobiology and Oxidative Stress

Simple Summary With the growing importance of climate change research, the biological effects of oxygen loss on marine biota remain understudied. Coral reefs support diverse marine organisms and provide valuable ecosystem services. Deoxygenation may induce mass coral mortality and reduce species richness on coral reefs. Corals exposed to nocturnal hypoxia may experience detrimental effects due to induced oxidative stress caused by the increase in reactive oxygen species (ROS). The stress interferes with the most fundamental biological processes vital for the symbiosis between corals and photosynthetic algae. In this study, the coral Acropora spp. exhibited evident injury in photosynthetic apparatus, de-epoxidation state and DNA. However, besides that, there were no signs of engaged antioxidant defense mechanisms against ROS or pigment degradation, which leads to a conclusion that these corals may be resilient to such oxygen daily oscillations. Nevertheless, while corals might survive such short-term abiotic stress, the growing number and intensity of hypoxic events across the global ocean may pose a massive threat to these keystone invertebrate species. Global ocean oxygen (O-2) 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 (Acropora spp.) to overnight hypoxic conditions (<2 mg/L of O-2). 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 Acropora 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.