Re-evaluating the Role of Symbiont-Produced Reactive Oxygen Species in Coral Bleaching

Coral bleaching continues to be the largest threat to vital reef ecosystems globally. The crisis has been strongly linked to external stressors, especially elevated water temperatures. Bleaching is the expulsion of algal symbionts by the coral host due to a breakdown in the symbiotic relationship. It is widely believed that when heat stressed, the photosynthetic process of algal symbionts is more prone to creating reactive oxygen species (ROS), which oxidatively damage coral tissue and thus trigger expulsion of the algae. However, there is also considerable evidence that is inconsistent with this theory. This study reevaluates the oxidative-stressor theory by attempting to replicate and expanding on previous experiments that were cited when the theory was initially proposed. The experiments reported here included a variety of reef-building corals, including species in the genera Acropora, Montipora, Pocillopora, and Pavona. Experiments were conducted in both controlled laboratory and outdoor environments. The effects of treatment with the antioxidants mannitol, ascorbate, and catalase on oxidative stress and bleaching were quantified at both heat-stress and control temperatures. Bleaching was quantified by measuring algal-symbiont density using flow cytometric counts, and oxidative stress was quantified using a malondialdehyde (MDA) assay. Statistical analysis used one-way ANOVA within each species to assess the effect of treatment on bleaching metrics. This was followed by pairwise comparisons between specific treatment groups. Antioxidant treatments were found to have no statistically significant effect on bleaching or oxidative stress. This suggests that photosynthetically produced ROS are not the primary explanation for why corals expel their symbionts when heat stressed. Re-examining the breakdown of this important symbiotic relationship is crucial to understanding and mitigating future bleaching events.