Organic ultraviolet filter mixture promotes bleaching of reef corals upon the threat of elevated seawater temperature

Global reef degradation is a critical environmental health issue that has triggered intensive research on ocean warming, but the implications of emerging contaminants in coral habitats are largely overlooked. Laboratory experiments assessing organic ultraviolet (UV) filter exposure have shown that these chemicals negatively affect coral health; their ubiquitous occurrence in association with ocean warming may pose great challenges to coral health. We investigated both short- (10-day) and long-term (60-day) single and co-exposures of coral nubbins to environmentally relevant organic UV filter mixtures (200 ng/L of 12 compounds) and elevated water temperatures (30 °C) to investigate their effects and potential mechanisms of action. The initial 10-day exposure of Seriatopora caliendrum resulted in bleaching only under co-exposure conditions (compounds + temperature). The 60-day mesocosm study entailed the same exposure settings with nubbins of three species (S. caliendrum, Pocillopora acuta and Montipora aequituberculata). Bleaching (37.5 %) and mortality (12.5 %) of S. caliendrum were observed under UV filter mixture exposure. In the co-exposure treatment, 100 % S. caliendrum and P. acuta bleached associating with 100 % and 50 % mortality, respectively, and significant increase of catalase activities in P. acuta and M. aequituberculata nubbins were found. Biochemical and molecular analyses indicated significant alteration of oxidative stress and metabolic enzymes. The results suggest that upon the adverse effects of thermal stress, organic UV filter mixture at environmental concentrations can cause bleaching in corals by inducing a significant oxidative stress and detoxification burden, suggesting that emerging contaminants may play a unique role in global reef degradation.