Medical authorities have spent years persuading people to use sunscreen to limit their exposure to UV light. But this effort was a bit of a failure, as there have been several places recently ban the use of sunscreen from beachgoers. These bans came into force after local waters were found to have high levels of some of the chemicals in the sunscreen, linked to the lower health of nearby coral reefs.

Several studies have shown that a specific ingredient in sunscreen, a chemical called oxybenzone, is at the root of the problem. But the mechanism by which oxybenzone can harm corals was unclear. Even without this understanding, it is difficult to say which sunscreens may pose a risk.

Researchers at Stanford University have now identified the problem. Corals turn oxybenzone from a chemical that can harmlessly absorb UV light into one that damages biological molecules after UV exposure. And there is evidence that bleaching corals makes things worse because corals are less able to withstand exposure.

This should not be a problem

Instead of working with slow-growing corals, the researchers did most of their work on their evolutionary relative, the anemone. And they started simply by confirming that oxybenzone is also a problem in these organisms, testing growth under different conditions. Healthy anemones exposed to a day-night cycle that includes UV light grow well. But add oxybenzone and it took a little over two weeks until all the anemones died.

However, it is strange that oxybenzone without a day-night cycle does not affect the survival of anemones. Both chemicals and UV light were needed to kill the animals. This result does not make much sense. We use oxybenzone as a sunscreen precisely because it manages to dissipate energy from UV radiation harmlessly. But in these animals, UV rays turned the chemical into a killer.

So researchers have suggested that oxybenzone is not the killer. Many chemicals, once in the cells, come into contact with enzymes that catalyze reactions with them, resulting in a related but different chemical. In some cases, this is because enzymes are used to detoxify a number of related chemicals. In other cases, it’s an incident caused by two chemicals that just look similar enough. Whatever the reason, the chemical that enters the cells may not be the chemical that changes the behavior of the cells (this often happens with drugs).

To find out if this is the case, the researchers exposed the anemones to oxybenzone for 18 hours, ground them and looked for any related chemicals in their contents. They found that most of the chemical ended up with glucose attached to it.

In test tubes, oxybenzone is not involved in any reactions that appear to damage biomolecules. But once glucose is attached, ultraviolet light causes the glucose-related form to chemically alter several biomolecules. And he did it catalytically, which means that none of the glucose-oxybenzone was consumed in the process. This means that it does not take much to cause significant damage.

It’s getting worse

While searching for the chemical derivatives of oxybenzone, the researchers noticed that much of the material was not in anemone cells; instead, it is found in symbiotic microorganisms associated with anemones. This finding suggests to some extent that the presence of symbionts protects anemones from the toxic effects of modified oxybenzone.

To confirm this, they turned to a species of coral that can undergo bleaching, which means the loss of microbial symbiotes. When present, the symbiotes absorbed enough glucose-oxybenzone to completely protect the corals from any deadly effects of UV radiation (in fact, any oxybenzone that remains unchanged probably provides some protection). But in the bleached version of the same coral, glucose-oxybenzone is deadly again. This result increases the risk that sunscreen is especially dangerous after bleaching corals.

Researchers suggest that all this is probably a major accident. The enzyme that adds glucose to this chemical probably evolved as a way to simply make toxins more soluble and thus easier to get rid of. And the fact that oxybenzone is great at absorbing UV light makes it a great sunscreen and is more likely to use that energy in a bad way once it’s modified.

The good news is that now that we have identified the mechanism in place, we have a better chance of finding other chemicals that could cause similar problems. This knowledge can allow us to design sunscreen products that are less likely to have these unexpected side effects.

Science, 2022. DOI: 10.1126 / science.abn2600 (Regarding DOI).

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