BEIJING, May 8 (Xinhua, Zhang Jiaxin) -- Coral reefs are among the most biologically rich and economically valuable ecosystems on Earth. Despite their immense environmental and socioeconomic value, coral reefs are in global decline due to a range of anthropogenic stressors, including rising sea temperatures, coastal nutrient pollution and overfishing. In recent years, hydroxybenzone, a chemical ingredient in topical sunscreens, has been identified as a threat to coral health. A study by Stanford University researchers, published in the journal Science on June 6, reveals how hydroxybenzophenone is accelerating the demise of this endangered ecosystem.

According to the National Park Service, as much as 6,000 tons of sunscreen (more than the weight of 50 blue whales) are discharged into coral reef areas in the United States each year. Scientists have long found that the organic compound hydroxybenzone in many sunscreens damages corals. As a result, sunscreens containing this compound have been banned in some coastal areas around the world.

However, the mechanism by which hydroxybenzophenone damages corals remains largely a mystery, making it difficult to ensure that sunscreen ingredients used as alternatives are safer for corals.

In the latest study, researchers examined sea anemones and bent mushroom corals (commonly known as mushroom corals). It was found that anemones exposed to hydroxybenzone in an environment simulating sunlight and artificial seawater all died within 17 days, while those exposed to hydroxybenzone in the absence of simulated sunlight still survived.

After absorbing UV light, hydroxybenzone is supposed to dissipate light energy in the form of heat and prevent sunburn. However, the unique metabolism of anemones and corals results in the formation of damaging free radicals from hydroxybenzophenone when exposed to sunlight. In other words, hydroxybenzophenone can make sunlight toxic to corals.

In addition, the researchers found evidence of a defense mechanism for corals against hydroxybenzophenone. Symbiotic algae in corals appear to protect their hosts by isolating the toxins brought on by coral-produced hydroxybenzophenones within themselves.

However, as seawater warms, corals expel their algal partners, exposing the bone-white coral skeleton. Thus, in addition to being more susceptible to disease and environmental shocks, this "bleached" coral is also more susceptible to damage from benzophenone without the protection of algae.

Researchers warn that benzophenone may not be the only sunscreen ingredient of concern. The metabolic pathway that converts hydroxybenzophenone into a toxin is also likely to apply to other common sunscreen ingredients with similar chemical structures.

Many sunscreens marketed as coral-friendly are based on metals such as zinc and titanium, rather than organic compounds such as hydroxybenzophenone. The researchers say that while these sunscreens are fundamentally different in function, it is unclear whether they are actually safer for corals, and they are planning to investigate the issue further.

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