A common but controversial sunscreen ingredient that is thought to harm corals might do so because of a chemical reaction that causes it to damage cells in the presence of ultraviolet light.
Researchers have discovered that sea anemones, which are similar to corals, make the molecule oxybenzone water-soluble by tacking a sugar onto it. This inadvertently turns oxybenzone into a molecule that — instead of blocking UV light — is activated by sunlight to produce free radicals that can bleach and kill corals. “This metabolic pathway that is meant to detoxify is actually making a toxin,” says Djordje Vuckovic, an environmental engineer at Stanford University in California, who was part of the research team. The animals “convert a sunscreen into something that’s essentially the opposite of a sunscreen”.
Oxybenzone is the sun-blocking agent in many suncreams. Its chemical structure causes it to absorb UV rays, preventing damage to skin cells. But it has attracted controversy in recent years after studies reported that it can damage coral DNA, interfere with their endocrine systems and cause deformities in their larvae2. These concerns have led to some beaches in Hawaii, Palau and the US Virgin Islands, banning oxybenzone-containing sunscreens. Last year, the US National Academies of Sciences, Engineering, and Medicine convened a committee to review the science on sunscreen chemicals in aquatic ecosystems; its report is expected in the next few months.
The latest study, published on 5 May in Science1, highlights that there has been little research into the potentially toxic effects of the by-products of some substances in sunscreens, says Brett Sallach, an environmental scientist at the University of York, UK. “It’s important to track not just the parent compound, but these transformed compounds that can be toxic,” he says. “From a regulatory standpoint, we have very little understanding of what transformed products are out there and their effects on the environment.”
But other factors also threaten the health of coral reefs; these include climate change, ocean acidification, coastal pollution and overfishing that depletes key members of reef ecosystems. The study does not show where oxybenzone ranks in the list.
To understand oxybenzone’s effects, Vuckovic, environmental engineer William Mitch at Stanford and their colleagues turned to sea anemones, which are closely related to corals, and similarly harbour symbiotic algae that give them colour.
The researchers exposed anemones with and without the algae to oxybenzone in artificial seawater, and illuminated them with light — including the UV spectrum — that mimicked the 24-hour sunlight cycle. All the animals exposed to both the chemical and sunlight died within 17 days. But those exposed to sunlight without oxybenzone or to oxybenzone without UV light lived.
Oxybenzone alone did not produce dangerous reactive molecules when exposed to sunlight, as had been expected, so the researchers thought that the molecule might be metabolized in some way. When they analysed anemone tissues, they found that the chemical bound to sugars accumulated in them, where it triggered the formation of oxygen-based free radicals that are lethal to corals. “Understanding this mechanism could help identify sunscreen molecules without this effect,” Mitch says.
The sugar-bound form of oxybenzone amassed at higher levels in the symbiotic algae than in the anemones’ own cells. Sea anemones lacking algae died around a week after exposure to oxybenzone and sunlight, compared with 17 days for those with algae. That suggests the algae protected the animals from oxybenzone’s harmful effects.
Corals that have been subject to environmental stressors such as changing temperatures often become bleached, losing their symbiotic algae. “If they’re weaker in this state, rising sea water temperature or ocean acidification might make them more susceptible to these local, anthropogenic contaminants,” Mitch says.
It’s not clear how closely these laboratory-based studies mimic the reality of reef ecosystems. The concentration of oxybenzone at a coral reef can vary widely, depending on factors such as tourist activity and water conditions. Sallach points out that the concentrations used in the study are more like “worst-case exposure” than normal environmental conditions.
The study lacks “ecological realism”, agrees Terry Hughes, a marine biologist at James Cook University in Townsville, Australia. Coral-bleaching events on Australia’s Great Barrier Reef, for example, have been linked more closely to trends in water temperature than to shifts in tourist activity. “Mass bleaching happens regardless of where the tourists are,” Hughes says. “Even the most remote, most pristine reefs are bleaching because water temperatures are killing them.”
Hughes emphasizes that the greatest threats to reefs remain rising temperatures, coastal pollution and overfishing. Changing sunscreens might not do much to protect coral reefs, Hughes says. “It’s ironic that people will change their sunscreens and fly from New York to Miami to go to the beach,” he says. “Most tourists are happy to use a different brand of sunscreen, but not to fly less and reduce carbon emissions.”