Medical authorities have spent years convincing people to use sunscreen to limit their exposure to UV rays. But that effort has had some setbacks, as several places have recently prohibits the use of sunscreen by swimmers. These bans came into effect after local waters were found to contain high levels of some of the chemicals found in sunscreen, which were associated with poorer health of nearby coral reefs.
Several studies have pointed to a specific component of sunscreen, a chemical called oxybenzone, to cause the problem. But the mechanism by which oxybenzone might harm corals was unclear. And, without that understanding, it’s hard to say which sunscreens might pose a risk.
Now researchers at Stanford University have identified the problem. Corals convert oxybenzone from a chemical that can harmlessly absorb UV light to one that damages biological molecules after being exposed to UV. And coral bleaching is proven to make matters worse, as coral is less able to withstand exposure.
That shouldn’t be a problem
Rather than working with corals, which are slow-growing, researchers have done 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 various conditions. Healthy anemones exposed to a day-night light cycle including UV light grew well. But add oxybenzone, and it took just over two weeks for all the anemones to die.
Surprisingly, however, oxybenzone without the day-night cycle did not affect anemone survival. It took both the chemical and the UV light to kill the animals. This result does not make much sense. We use oxybenzone as a sunscreen precisely because it manages to dissipate the energy of UV radiation in a harmless way. Yet 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 but distinct chemical. In some cases, this is because the enzymes are used to detoxify a range of related chemicals. In other cases, it is an accident caused by two chemicals that are sufficiently similar. Whatever the reason, the chemical that enters cells may not be the one that changes cell behavior (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, crushed them, and looked for any related chemicals in their contents. Most of the chemicals, they found, had ended up with glucose attached to them.
In test tubes, oxybenzone does not engage in any reactions that appear to damage biomolecules. But once the glucose is attached, UV light causes the glucose-bound form to chemically alter a few biomolecules. And it did it catalytically, meaning no glucose oxybenzone was consumed in the process. That means it doesn’t take much to do massive damage.
It’s getting worse
Looking for the chemical derivatives of oxybenzone, the researchers noticed that much of the material was not in the cells of the anemone; instead, it has been found in the symbiotic microorganisms associated with the anemone. This discovery 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 species of coral that can experience bleaching, i.e. the loss of its microbial symbionts. When present, the symbionts absorbed enough glucose-oxybenzone to completely protect the coral from any lethal effects of UV rays (in fact, any unmodified oxybenzone probably offers some protection). But in a bleached version of the same coral, glucose-oxybenzone is deadly again. This result increases the risk that sunscreen will be particularly dangerous following a coral bleaching event.
The researchers suggest it was all probably a big accident. The enzyme that adds glucose to this chemical likely evolved to simply make toxins more soluble and therefore easier to eliminate. And the fact that Oxybenzone is great at absorbing UV light makes it a great sunscreen and more likely to use that energy in unfortunate ways once it’s altered.
The good news is that now that we’ve identified the mechanism involved, we have a better chance of detecting other chemicals that could be causing similar issues. This knowledge could allow us to design sunscreens that are less likely to have these unexpected side effects.
Science, 2022. DOI: 10.1126/science.abn2600 (About DOIs).
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