Sunscreens released into the water by bathers are known to damage corals. Now, research reveals a surprising mechanism behind this effect: a typical sunscreen ingredient turns into a light-activated toxin in coral polyps. The results show that this phototoxin is especially harmful to corals that are already affected by so-called bleaching. Scientists say these findings could now help develop coral-safe sunscreens.
Fascinating underwater gardens with far-reaching significance: Coral reefs are among the most species-rich habitats on earth and play an important role in the complex processes that occur in the oceans – and therefore for us humans as well. But these precious ecosystems are under threat: some man-made stressors are leading to massive losses worldwide. First of all, rising sea water temperatures as a result of climate change are destroying coral polyps responsible for building reefs. They lead to so-called stinging whitening, 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 bathers of tourists can damage corals, research shows.
Following the problem
The organic compound oxybenzone, which provides UV protection in many sunscreen creams, has already been identified as the responsible ingredient. For this reason, in some regions with coral reefs, efforts have already been made to limit the use of creams containing this ingredient. However, the mechanisms by which oxybenzone causes its damage remain largely unclear. This makes it difficult to ensure that the sunscreens proposed as alternatives are actually safer for the corals.
Therefore, scientists, led by William Mitch from Stanford University, investigated the chemical and biological mechanisms of coral damage caused by oxybenzone. They used sea anemones and a species of coral as models in their experiments. They supplied them with oxybenzone through the water in their aquariums, and also exposed them to various types of light irradiation. The team then examined the effects of the treatments using a series of analytical techniques.
This revealed a key factor in light: anemones and corals exposed to oxybenzone in full-spectrum light died within 17 days. However, polyps exposed to oxybenzone remained viable in the absence of light. “It was amazing to see that oxybenzone was making sunlight harmful to the corals because it’s the exact opposite of what it does to us,” says Mitch.
What protects us destroys the corals
As the researchers explain, oxybenzone is so popular in sunscreens because it absorbs ultraviolet light very efficiently and turns it into harmless heat. But the stingers apparently have a problematic effect, as shown by the results of scientists’ research: in them, oxybenzone is transformed by metabolic processes into a substance which, under the influence of sunlight, creates harmful radicals. Research results show that the sunscreen substance is converted into a phototoxin that is harmful to corals.
The researchers were also able to show that there seems to be another important factor in the problem: damage depends on how well the corals are equipped with symbiosis partners. Because small algae living in polyps can capture phototoxin and thus protect their hosts, according to research results. Scientists emphasize that this discovery is also of great importance. Because it is known that corals lose their algae partners as part of bleaching caused by excessive water temperature. According to research results, such “bleached” corals are not only more susceptible to disease and environmental influences, but also to the harmful effects of oxybenzone from sunscreen.
“I hope our research will help pave the way to the development of coral-safe sunscreens,” says lead author Djordje Vuckovic of Stanford University. According to the researchers, it should be noted that other UV protective substances with a chemical structure similar to that of oxybenzone can also form phototoxic metabolites. In summary, Vuckovic says, “It would be ironic if even ecotourism to protect coral reefs exacerbated their decline.”
Source: Stanford University, article in Science, doi: 10.1126 / science.abn2600