Health authorities have spent years persuading people to use sunscreen to limit their exposure to UV light. However, the impact of this effort is waning somewhat, with some places recently banning beachgoers from using sunscreen. These bans came into effect after high levels of certain chemicals in sunscreen used on local beaches were found to be related to the health status of nearby coral reefs.
Several studies on the subject have shown that the root of the problem is a chemical called oxybenzone, one of the ingredients in sunscreen. However, it has not been clearly determined how oxybenzone damages the corals. Without this damage mechanism, it is not possible to tell which sunscreens may pose a risk.
Now, researchers from Stanford University seem to have pinpointed the problem. Corals convert oxybenzone from a chemical that can harmlessly absorb UV light to a chemical that damages biological molecules after exposure to UV. And there is evidence that coral bleaching makes things worse as corals become less resistant to exposure.
Instead of working with slow-growing corals, the researchers did most of their work on its evolutionary relative, the anemone. And they started by testing growth under a variety of conditions, confirming that oxybenzone was also a problem in these organisms. Healthy anemones exposed to a day-night light cycle that included UV light grew well. But when oxybenzone was added, it took a little over two weeks for all the anemones to die.
But strangely, in the absence of the day-night cycle, oxybenzone did not affect the anemones’ ability to survive. For the death of these creatures, both the presence of the chemical and the presence of UV light were required. Considering that we used oxybenzone as a sunscreen due to its ability to harmlessly dissipate the energy from UV radiation, this conclusion didn’t seem very plausible. However, in these creatures, UV turns the chemical into a killer.
After many chemicals enter cells, other chemicals can react with enzymes or directly because of their similarity to another chemical. So to see if that was the case, the researchers exposed the anemones to oxybenzone for 18 hours and then looked for the relevant chemicals in their ingredients. Most of the chemical they found indicated it was glucose bound to oxybenzone.
In test tubes, oxybenzone does not undergo any reaction that appears to damage biomolecules. But after the glucose binds, UV light causes the glucose-bound form to chemically alter several biomolecules. And this reaction was catalytic, meaning no glucose-oxybenzone was consumed in the process. So, it didn’t take much time for serious damage to be inflicted.
While searching for chemical derivatives of oxybenzone, researchers noticed that most of the substance was not in anemone cells; instead, it was involved in symbiotic microorganisms associated with the anemone. To see how well the effects of symbiont protect these creatures, they turned to a type of coral that could undergo bleaching and found that the symbiont protected the coral, but in the bleached derivative, oxybenzone became deadly again.
Fortunately, now that this death mechanism is found, it will be easier for us to find chemicals that may have similar effects, and it will probably be possible for us to take appropriate steps to protect such creatures from similar effects.
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