BioEd Online

Sunscreen can bleach coral reefs, researchers have confirmed. The chemicals that filter ultraviolet (UV) light can activate latent viral infections in the symbiotic microalgae that the corals rely on for nutrition.

Many divers are already warned not to wear sunscreen near corals, but usually for the general reason that introducing foreign chemicals into the water is a bad idea. The new study puts some scientific evidence behind this precautionary approach.

Resort managers in Mexico first spotted the trouble when enclosed pools called cenotes on the Yucatan Coast became popular swimming holes. "They saw a high mortality of all living things," says marine biologist Roberto Danovaro from the Polytechnic University of Marche in Ancona, Italy. Concerned that sunblock might be the culprit, several resorts banned its use by snorklers and divers exploring the cenotes and nearby reefs.

Danovaro and his co-workers set out to see whether they could prove the link between sunscreen and die-offs. They collected nubbins of coral from reefs scattered throughout the tropics: the Caribbean Sea off Mexico, the Indian Ocean off Thailand, the Red Sea off Egypt and the Pacific Ocean near Indonesia. When they incubated each sample in seawater spiked with as little as 10 microlitres of sunscreen per litre, coral bleaching occurred within four days. Controls incubated in plain seawater remained healthy, the team reports in a forthcoming issue of Environmental Health Perspectives¹.

Viral attack

Samples of water drawn after 18-48 hours were full of symbiotic algae that had detached from the coral nubbins. Instead of a healthy brownish green, the loose algae were pale or transparent and punched full of holes. Viral particles were abundant as well, suggesting that the algae or coral harboured a latent infection that was activated by something in the sunscreens. "What was surprising to us was that the same latent infection was found in [corals from] so many places — all over the world," Danovaro says.

Danovaro and his team tested additional samples with several chemical components of suncreen and found that three UV-filtering chemicals (a cinnamate, a benzophenone and a camphor derivative) as well as butyl paraben, a preservative, caused the release of viral particles and bleached the coral. The other chemicals tested from the creams had no effect.

"I'm pretty convinced that viruses are instrumental in the whole bleaching process,” says William Wilson, who studies marine viruses at the Bigelow Laboratory for Ocean Sciences in Boothbay Harbor, Maine, and was not involved in this work. Wilson and his colleagues have recently demonstrated that UV light can similarly prompt viral attacks². "Coral bleaching seems to happen when the corals are stressed,” he says. Attacking a weakened host is “a very classic response of a virus”, he adds.

Some biologists have questioned whether effort should be expended on pinning down the effects of sunscreens, when warming waters and acidifying oceans clearly threaten more coral than do vacationers, who visit only about 10% of the world’s reefs. Others counter that the threat from sunscreen is the easiest to control. "I'm not suggesting anyone should get burned," Danovaro says, "just that they use a physical sunscreen instead" — such as one based on titanium dioxide, or just a t-shirt.