Words by: Ker Than for National Geographic News
The sunscreen that you dutifully slather on before a swim on the beach may be protecting your body—but a new study finds that the chemicals are also killing coral reefs worldwide.
Four commonly found sunscreen ingredients can awaken dormant viruses in the symbiotic algae called zooxanthellae that live inside reef-building coral species.
The chemicals cause the viruses to replicate until their algae hosts explode, spilling viruses into the surrounding seawater, where they can infect neighboring coral communities.
Zooxanthellae provide coral with food energy through photosynthesis and contribute to the organisms' vibrant color. Without them, the coral "bleaches"—turns white—and dies.
"The algae that live in the coral tissue and feed these animals explode or are just released by the tissue, thus leaving naked the skeleton of the coral," said study leader Roberto Danovaro of the Polytechnic University of Marche in Italy.
The researchers estimate that 4,000 to 6,000 metric tons of sunscreen wash off swimmers annually in oceans worldwide, and that up to 10 percent of coral reefs are threatened by sunscreen-induced bleaching.
The study appeared online in the journal Environmental Health Perspectives.
Danovaro and his team studied the effects of sunscreen exposure on coral samples from reefs in the Pacific, Atlantic, and Indian Oceans.
Even low levels of sunscreen, at or below the typical amount used by swimmers, could activate the algae viruses and completely bleach coral in just four days, the results showed.
Seawater surrounding coral exposed to sunscreen contained up to 15 times more viruses than unexposed samples.
Several brands of popular sunscreens were tested and all had four ingredients in common: paraben, cinnamate, benzophenone, and a camphor derivative.
Robert van Woesik, a coral expert at the Florida Institute of Technology, was not involved in the research. He questions whether conditions in the study accurately reflect those found in nature.
For example, the coral samples were exposed to sunscreen while in plastic bags to avoid contaminating the reefs. But van Woesik worries this prevented dilution of the chemicals through natural water circulation.
"Under normal situations on a coral reef, corals would not be subjected to these high concentrations because of rapid dilution," van Woesik said.
But according to study author Danovaro, the effect is not dose dependent—so coral's exposure to a very small dose of sunscreen is just as dangerous as a high exposure.
"It is more like on-off," he said. "Once the viral epidemic is started, it is not a problem of toxicity."
Rebecca Vega Thurber, a marine virus and coral researcher at San Diego State University in California, said the new results are further evidence of an alarming trend.
"Other [human-induced] factors such as coastal pollution, overfishing, and sedimentation all contribute to coral reef habitat degradation, and this work continues in that vein," said Vega Thurber, who was also not involved in the research.
"But before we ban sunscreens, we must first determine if local ambient concentrations of sunscreens are positively correlated with coral bleaching events."
Danovaro says banning sunscreen won't be necessary, and points out two simple things swimmers can do to reduce their impact on coral: Use sunscreens with physical filters, which reflect instead of absorb ultraviolet radiation; and use eco-friendly chemical sunscreens.
Australian researchers are also working to develop a sunscreen based on a natural ultraviolet-blocking compound found in coral.