Irradiance dependency of oxidative stress and coral bleaching

The degradation of tropical coral reefs continues unabated as global climate change causes mass “coral bleaching” events. At the organismal level, there is significant evidence that the production of reactive oxygen (ROS) and nitrogen (RNS) species, followed by programmed cell death, causes coral bleaching. Additionally, in tropical marine environments exposure to high irradiances of solar radiation contributes to the photooxidative production of ROS in corals. But most thermal stress experiments on corals have not manipulated and tested the direct and interactive effects of solar radiation on coral bleaching, which is further compounded by the ecologically unrealistic low irradiances used in many experiments. Using published data, a direct relationship between excitation pressure (Q_m) on photosystem II for the photosymbionts of corals with irradiance, when exposed to thermal stress, is demonstrated here. Using these results, the photoinhibition model of oxidative stress and coral bleaching is refined for shallow coral reefs; high irradiances increase Q_m and when exposed to thermal stress result in an increase in ROS and coral bleaching, but under low irradiances the production of ROS decreases while the production of RNS increases, leading to coral bleaching. This suggests that irradiance-dependent effector molecules of coral bleaching (i.e., ROS versus RNS) may dominant the oxidative stress landscape in the coral holobiont. Incorporating measurements of irradiance, tissue oxygen concentrations and ROS/RNS levels, in addition to temperature, into experiments and predictive models is required in order to better understand the full range of environmental conditions that cause coral bleaching.