Sea anemones, which are closely related to corals, and mushroom coral can turn oxybenzone — a chemical that protects people against ultraviolet light — into a deadly toxin that’s activated by light. The good news is that algae living alongside the creatures can soak up the toxin and blunt its damage, researchers report in the May 6 Science.
But that also means that bleached coral reefs lacking algae may be more vulnerable to death. Heat-stressed corals and anemones can eject helpful algae that provide oxygen and remove waste products, which turns reefs white. Such bleaching is becoming more common as a result of climate change (Science News 4/7/20).
When you swim with sunscreen on, chemicals like oxybenzone can seep into the water, where they’re absorbed by corals.
Currently, somewhere between 4,000 and 6,000 tons of sunscreen enters coral reef areas around the world each year, according to the U.S. National Park Service. That's a lot of sunscreen considering how little it takes to cause toxic effects.
According to previous research, toxicity occurs at a concentration of 62 parts per trillion. That's the equivalent of a drop of water in an Olympic swimming pool, according to Omri Bronstein, a researcher at Tel Aviv University.
The findings hint that sunscreen pollution and climate change combined could be a greater threat to coral reefs and other marine habitats than either would be separately, says Craig Downs. He is a forensic ecotoxicologist with the nonprofit Haereticus Environmental Laboratory in Amherst, Va., and was not involved with the study.
Previous work suggested that oxybenzone can kill young corals or prevent adult corals from recovering after tissue damage. As a result, some places, including Hawaii and Thailand, have banned oxybenzone-containing sunscreens. These substances contain nanoparticles that can disrupt coral’s reproduction and growth cycles, ultimately leading to bleaching.
In the new study, environmental chemist Djordje Vuckovic of Stanford University and colleagues found that glass anemones (Exaiptasia pallida) exposed to oxybenzone and UV light add sugars to the chemical. While such sugary add-ons would typically help organisms detoxify chemicals and clear them from the body, the oxybenzone-sugar compound instead becomes a toxin that’s activated by light.
Anemones exposed to either simulated sunlight or oxybenzone alone survived the length of the experiment, or 21 days, the team showed. But all anemones exposed to fake sunlight while submersed in water containing the chemical died within 17 days.
The anemones’ algal friends absorbed much of the oxybenzone and the toxin that the animals were exposed to in the lab. Anemones lacking algae died days sooner than anemones with algae.
In similar experiments, algae living inside mushroom coral (Discosoma sp.) also soaked up the toxin, a sign that algal relationships are a safeguard against its harmful effects. The coral’s algae seem to be particularly protective: Over eight days, no mushroom corals died after being exposed to oxybenzone and simulated sunlight.
Non-nano zinc oxide and non-nano titanium dioxide are the only coral-safe sunscreen filters and they are safe and effective in blocking UV rays. Sunscreens formulated with non-nano particles will clearly state "non-nano zinc oxide and/or non-nano titanium dioxide on the products active ingredient list".