Up to 6,000 tons of sunscreen — more than the weight of 50 blue whales — wash through U.S. reef areas every year, according to the National Park Service. Scientists have known for some time that oxybenzone, an organic compound found in many sunscreens, can damage corals. As a result, sunscreens with this compound have been banned in the U.S. Virgin Islands and Hawaii, the island nation of Palau, and Bonaire, an island municipality of the Netherlands, among other places.
However, the mechanisms by which oxybenzone does harm have largely remained a mystery, making it difficult to ensure that sunscreen components proposed as alternatives are truly safer for corals.
Protection for humans, damage for corals
In their new study, Mitch, Pringle, Vuckovic, and other Stanford researchers used anemones as surrogates for corals, which are harder to experiment with, as well as mushroom corals. Exposed to oxybenzone in artificial seawater under simulated sunshine, the anemones all died within 17 days, whereas anemones exposed to oxybenzone in the absence of simulated sunlight remained viable.
After absorbing ultraviolet light, oxybenzone is designed to dissipate the light energy as heat, preventing sunburn. The anemones and corals, however, metabolized oxybenzone in such a way that the resulting substance formed damaging radicals when exposed to sunlight.
In addition to this vulnerability, the researchers found evidence for a coral defense mechanism. Symbiotic algae in corals appeared to protect their hosts by sequestering within themselves the toxins that corals produced from oxybenzone.
Ensuring sunscreens are safe for corals and other marine species
Oxybenzone may not be the only sunscreen ingredient of concern, the researchers warn. The same metabolic pathways that appear to convert oxybenzone into a potent toxin for corals may do something similar with other common sunscreen ingredients, many of which share similar chemical structures and so could form similar phototoxic metabolites.
Co-authors of the study also include Amanda Tinoco, a research technician at Stanford School of Medicine at the time of the research; Lorraine Ling, a postdoctoral scholar in genetics at the time of the research; and Christian Renicke, a postdoctoral research scientist in genetics.
The research was funded by the Stanford Woods Institute for the Environment’s Environmental Venture Projects program and the National Science Foundation.
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