Researchers have long known that oxybenzone, a chemical found in sunscreen, is harmful to corals. But exactly why and how it affects these animals has remained a mystery.
Now scientists at Stanford University have discovered that when exposed to sunlight, anemones—which are closely related to corals—make oxybenzone water-soluble by combining it with sugar, but this process also makes the chemical toxic to the animal. They published their findings in the journal Science.
Corals live symbiotically with algae. However, when subject to high temperatures or other stressors, the marine invertebrates expel the algae and turn completely white. This process, called coral bleaching, leaves the animals vulnerable to mortality.
To figure out how oxybenzone harms anemones, the researchers tested the effects of the chemical and sunlight on anemones with their algae and without it. In an artificial environment, they found that all anemones exposed to oxybenzone and sun died within 17 days, while those exposed to only oxybenzone survived.
“It was strange to see that oxybenzone made sunlight toxic for corals—the opposite of what it is supposed to do,” study author William Mitch says in a statement. “The compound is good at absorbing light within the waveband we tested, which is why it’s so common in sunscreens.”
When exposed to sunlight, anemones modify oxybenzone within their cells by attaching glucose to it, which turns it into a toxin.
“This metabolic pathway that is meant to detoxify is actually making a toxin,” Djordje Vuckovic, a Stanford environmental engineer who was part of the research team, tells Nature's Jyoti Madhusoodanan. They “convert a sunscreen into something that’s essentially the opposite of a sunscreen.”
The symbiotic algae seemed to offer some protection against this process. Anemones without algae survived for only a week, while those with it survived for more than two weeks. This suggests that “the algae protect the hosts by sequestering the phototoxins. Thus, oxybenzone may be particularly phototoxic to bleached corals, exacerbating the damage due to global stressors,” write the authors.
Because of its harmful impacts on corals, movements to ban suncreens containing oxybenzone have sprung up across the world. In 2018, Hawaii became the first state to pass legislation banning oxybenzone-containing sunscreens, which went into effect in 2021. Other jurisdictions worldwide have since followed suit. In recent years, scientists have been working on more environmentally-friendly alternatives, including sunscreens containing compounds from algae, seaweed and cyanobacteria.
Whether this lab study matches what happens in an actual reef ecosystem is unknown—the authors created a “worst-case exposure” scenario, rather than one that mimics normal environmental conditions, Brett Sallach, an environmental scientist at the University of York, U.K. tells Nature.
Terry Hughes, a marine biologist at James Cook University in Townsville, Australia, tells the publication that bigger threats to corals are climate change, pollution and overfishing. Those three factors have contributed to the loss of half of the world’s reefs since the 1950s.
“Mass bleaching happens regardless of where the tourists are. Even the most remote, most pristine reefs are bleaching because water temperatures are killing them,” he tells Nature. “It’s ironic that people will change their sunscreens and fly from New York to Miami to go to the beach. Most tourists are happy to use a different brand of sunscreen, but not to fly less and reduce carbon emissions.”