For years, medical authorities have convinced people to use sunscreen to limit their exposure to UV light. But that effort has met with a bit of a setback, as several locations have recently banned the use of sunblocks by beachgoers. Those bans came into effect after local waters were found to contain high levels of some of the chemicals in sunblock, which have been associated with the diminished health of nearby coral reefs.
Several studies have shown that one specific component of the sunblock, a chemical called oxybenzone, is at the root of the problem. But the mechanism by which oxybenzone might harm corals wasn’t clear. And without that understanding, it’s hard to say which sunblocks might pose a risk.
Now researchers at Stanford University have identified the problem. The corals convert oxybenzone from a chemical that can harmlessly absorb UV light into one that damages biological molecules after UV exposure. And there is some evidence that coral bleaching makes things worse because the coral is less resistant to exposure.
This shouldn’t be a problem
Rather than working with corals, which grow slowly, the researchers did most of their work on its evolutionary relative, the anemone. And they started simply by confirming that oxybenzone was also a problem in these organisms by testing growth under different conditions. Healthy anemones exposed to a day/night light cycle with UV light grew well. But add oxybenzone and it took a little over two weeks for all the anemones to die.
Strangely enough, however, without the day-night cycle, oxybenzone did not affect the survival of the anemones. It took both the chemical and UV light to kill the animals. This result is not very logical. We use oxybenzone as a sunblock precisely because it manages to dissipate the energy of UV radiation harmlessly. But in these animals, UV turned the chemical into a killer.
So the researchers hypothesized that oxybenzone was not the killer. Many chemicals, once inside cells, come into contact with enzymes that catalyze reactions with them, resulting in a related yet distinct chemical. In some cases, this is because the enzymes are used to detoxify a range of related chemicals. In other cases, it’s an accident caused by two chemicals that are just enough alike. Whatever the reason, the chemical that gets into the cells may not be the chemical that changes the behavior of the cells (this is often the case with drugs).
To find out if that was the case here, the researchers exposed anemones to oxybenzone for 18 hours, ground them up and looked for any related chemicals in their contents. Most of the chemical, they found, had ended up with glucose attached to it.
In test tubes, oxybenzone does not take on reactions that appear to damage biomolecules. But once the glucose is attached, UV light triggers the glucose-linked form to chemically alter some of the biomolecules. And it did so catalytically, meaning none of the glucose oxybenzone was consumed in the process. That means it doesn’t take much to do significant damage.
Getting worse
While looking for the chemical derivatives of oxybenzone, the researchers noticed that much of the material was not inside the anemone cells; instead, it was found in the symbiotic microorganisms associated with the anemone. This finding suggested to some extent that the presence of the symbionts protected the anemones from the toxic effects of the modified oxybenzone.
To confirm this, they turned to a coral species that can undergo bleaching, meaning the loss of its microbial symbiotes. When present, the symbionts ingested enough of the glucose oxybenzone to completely protect the coral from the deadly effects of UV radiation (in fact, any oxybenzone left unchanged will likely provide some protection). But in a bleached version of the same coral, the glucose oxybenzone is deadly again. This result raises the risk that sunscreen is particularly dangerous in the aftermath of a coral bleaching event.
The researchers suggest that this is probably all a major accident. The enzyme that adds the glucose to this chemical probably evolved as a way to simply make toxins more soluble and thus easier to remove. And the fact that oxybenzone is good at absorbing UV light makes it a great sunscreen, and it’s more likely to use that energy in unfortunate ways once it’s adjusted.
The good news is that now that we’ve identified the mechanism, we’re more likely to detect other chemicals that could cause similar problems. That knowledge could allow us to design sunscreens that are less likely to cause these unexpected side effects.
Science, 2022. DOI: 10.1126/science.abn2600 (About DOIs).
