Photobiological Effects of Sunscreens on Scleractinian Coral, Acropora yongei

Coral reefs are home to a quarter of all marine species, provide physical protection to coastlines, and generate several billion dollars each year with tourism. Heavy tourism brings an increase in the use of personal care products, such as sunscreen, which often ends up contaminating the ocean waters. We evaluated the severity of toxicity effects from sunscreens to corals and how their dilutions affect coral physiological health. Multiple bioreporters of health were used, from coral color, to coral (zooxanthellae) photoefficiency, and coral fluorescence ability. This last parameter indeed originates from green fluorescent proteins (GFPs), an antioxidant and photo‐protectant expressed by coral tissue to manage the photobiology of their symbiotic dinoflagellates. In this study, the coral Acropora yongei was exposed to three individual sunscreen brands (SB) at various dilutions (10 −9 to 10 −12 times relative to the straight off‐the‐tube lotion) under controlled experimental conditions in aquaria over a five‐week period. SB #1 contained oxybenzone, homosalate, and octisalate as active ingredients, SB #2 contained titanium dioxide and zinc oxide, and SB #3, considered to be “reef‐safe” contained mainly zinc oxide. The exposure consisted of short term (6 hrs/day) exposures to the various SBs, during weekdays for five weeks (weekends had no exposure). Following this exposure period experiment, a recovery period analysis (no exposure to sunscreen) lasted for five weeks. Results showed bleaching (based on coral color) of nearly all coral replicates exposed to all sunscreen treatments. Analysis of photobiological performance showed no strong differences between experimental conditions of exposure and recovery with the exposure removed (photoefficiency of PSII was not affected). Pre‐dawn and midday analysis revealed that stress was similar during daytime and nighttime. Since photosynthesis is performed by endosymbiotic zooxanthellae only, we can conclude that the zooxanthellae were not the source of stress causing the bleaching; they thus seem unaffected by the treatments, in agreement with the PSII photoefficiency data. Results specific to the coral host only suggest that sunscreen compounds enter the coral tissue and generate oxidative stress which is demonstrated with the changes in fluorescence that occurred during the exposure period. Coral fluorescence drastically decreased from Day 0 of the experiment until the end of the recovery period for all sunscreen brands. Exposure to oxybenzone, the main synthetic chemical used in sunscreen, and titanium dioxide and zinc oxide based sunscreen, generated an irreversible decrease in coral fluorescence. Unexpectedly, exposure to nanoparticle‐based zinc oxide sunscreen, thought to be safer for corals and marine ecosystems, induced similar irreversible changes. Sunscreen is beginning to become a threat to the resiliency of coral reefs to climate change and it is therefore necessary to understand how it affects the holobionts of corals. This study provides a basis for broader research interest on providing safe guidelines for human exploration of nature with minimal impact on the organisms we need to protect. Support or Funding Information Project received financial support from the BEST Initiative ( https://deheynlab.ucsd.edu/best‐2/ ).