The interactive impacts of a constant reef stressor, ultraviolet radiation, with environmental stressors on coral physiology

Reef-building corals create one of the most biodiverse and economically important ecosystems on the planet. Unfortunately, global coral reef ecosystems experience threats from numerous natural stressors, which are amplified by human activities. One such threat is ultraviolet radiation (UVR) from the sun; a genotoxic stressor that is a double-edged sword for corals as they rely on sunlight for energy. More intense UVR has been shown to have greater direct impacts on animal physiology, and these may be exacerbated by co-occurring stressors. The aim of this systematic literature review was to examine if the same applies to corals; that is, if the co-exposure of a constant stressor (UVR) with other stressors has a greater impact on coral physiology than if these stressors occurred separately. We reviewed the biochemical and cellular processes impacted by UVR and the defenses corals have against UVR. The main stressors investigated with UVR were temperature, nitrate, nutrient, oil, water motion, and photosynthetically active radiation (PAR). UVR generally worsened the physiological impacts of other stressors (e.g., by decreasing zooxanthellae and chlorophyll densities). There were species-specific differences in their tolerance to UVR (differences in zooxanthellae populations, sunscreen production and depth) and environmental stress (e.g., resilience to some oils), and that ambient levels of UVR were often beneficial (i.e., nullifying impacts of nitrates). We highlight areas of future investigation including examining and assessing other interacting stressors and their impacts (e.g., ocean acidification, ocean deoxygenation, toxins and pollutants), investigating impacts of multiple stressors with UVR on the coral microbiome, and elucidating the effects of multi-stressors with UVR across early-life history stages (especially larvae). UVR is a pervasive stressor to corals and can interact with other environmental conditions to compromise the resilience of corals. This environmental driver needs to be more comprehensively examined alongside climate change stressors (e.g., temperature increases, ocean acidification and hypoxia) to better understand future climate scenarios on reefs.

Automated summary

Reef-building corals, one of the most biodiverse and economically important ecosystems on the planet, are threatened by various natural stressors. Among them, ultraviolet radiation (UVR) has a double-edged impact on corals as it is a genotoxic stressor but also essential for their energy production. This systematic literature review investigates the physiological impacts of co-exposure to UVR and other stressors on corals. The results show that UVR generally exacerbates the negative effects of other stressors on corals. There are species-specific differences in tolerance to UVR and other environmental stressors, and the ambient levels of UVR can be beneficial. Future research should explore the interactions between UVR and other stressors, investigate the effects of multiple stressors on the coral microbiome, and examine the impacts of multi-stressors with UVR across early-life history stages.