Sunscreen that bathers release into the water is known to damage coral. Now a study reveals the surprising mechanism behind this effect: A typical component of sunscreen turns into a light-activated toxin in coral polyps. The results show that this phototoxin is particularly harmful to corals, which are already affected by what is known as bleaching. These findings could now help to develop coral-safe sunscreens, the researchers say.
Fascinating underwater gardens with far-reaching importance: Coral reefs are among the most species-rich habitats on earth and play an important role in the complex processes in the oceans – and therefore also for us humans. But these precious ecosystems are under threat: Some man-made stress factors are leading to severe losses worldwide. Above all, the rising seawater temperatures as a result of climate change are damaging the coral polyps responsible for building the reefs. They lead to the so-called bleaching of the cnidarians, which can lead to the death of large reef areas. Another problematic effect has been known for some time: sunscreens that are released into the water from the skin of bathing tourists can damage the corals, studies show.
On the trail of the problem
The organic compound oxybenzone, which provides UV protection in many sunscreens, has already been identified as the responsible ingredient. For this reason, efforts have already been made in some regions with coral reefs to limit the use of creams containing this ingredient. However, the mechanisms by which oxybenzone causes harm have remained largely obscure. This makes it difficult to ensure that the sunscreens proposed as alternatives are actually safer for corals.
That is why the scientists led by William Mitch from Stanford University have now investigated the chemical and biological mechanisms by which oxybenzone damages corals. In their experiments, they used a sea anemone and a coral species as models. They supplied these cnidarians with oxybenzone via the water in aquariums and also exposed them to different types of light irradiation. Through a series of analytical techniques, the team then examined the effects produced by the treatments.
This revealed the crucial factor of light: the anemones and corals exposed to oxybenzone under full-spectrum light died within 17 days. However, the polyps exposed to oxybenzone in the absence of light remained viable. “It was amazing to see that oxybenzone appears to be making sunlight harmful to corals, because that’s the exact opposite of what it does to us,” says Mitch.
What protects us damages corals
As the researchers explain, oxybenzone is so popular in sunscreens because it absorbs ultraviolet light very effectively and converts it into harmless heat. But the cnidarians apparently have a problematic effect, as the researchers’ study results show: in them, oxybenzone is converted by metabolic processes into a substance that forms harmful radicals when exposed to sunlight. The study results show that the light protection substance in sun creams is converted in corals into a phototoxin that is harmful to them.
The researchers were also able to show that there is apparently another important factor in the problem: the damage depends on how well the corals are equipped with their symbiosis partners. Because the small algae that live in the polyps can intercept the phototoxin and thus protect their hosts, according to the test results. The researchers emphasize that this finding is also of great importance. Because it is known that corals lose their algae partners as part of the bleaching caused by excessive water temperatures. According to the study results, such “bleached” corals are not only more susceptible to diseases and environmental influences, but also to the harmful effects of the oxybenzone from the sun creams.
“I hope that our research will help pave the way for the development of coral-safe sunscreens,” says lead author Djordje Vuckovic of Stanford University. According to the researchers, it is important to note that other UV protection substances with similar chemical structures to oxybenzone could also form phototoxic metabolites. In conclusion, Vuckovic says, “It would be sadly ironic if even ecotourism, which is supposed to protect coral reefs, were to exacerbate their decline.”
Source: Stanford University, Article: Science, doi: 10.1126/science.abn2600