Interactive effects of ultraviolet radiation and thermal stress on two reef-building corals

In shallow tropical waters, ultraviolet radiation (UVR) occurs at high intensity simultaneously with high water temperatures, and both stressors are predicted to increase in the future and to have a major impact on reef coral survival. The poor knowledge of the interactive effects of those two factors, however, prevents a good estimation of the coral resistance to climate change pressure. The results obtained in this study on two genetically distant scleractinian coral species, Pocillopora damicornis and Turbinaria reniformis, highlight an interaction of temperature and UVR on the corals' physiology as well as a species-specific response. Overall, increase in UVR level exacerbated the negative impact of thermal stress. Whereas stressors did not reduce the autotrophic capacity (symbiont density, net photosynthetic rates) of T. reniformis, they significantly lowered it in P. damicornis, which initially contained twice more symbionts in its tissue. Only grazing rates (heterotrophy) were significantly decreased in T. reniformis under thermal stress. For both corals, calcification was slowed by the combination of thermal and UVR stress. Calcification was impaired likely due to a decrease in autotrophic energy supply in P. damicornis, and in heterotrophic energy supply in T. reniformis. This study confirms that the response of corals to global change needs to be studied using multifactorial approaches and a combination of different environmental factors.