Sunscreen bottles are often labeled “reef friendly” and “coral safe.” These claims usually mean that the lotions have replaced oxybenzone, a chemical that can harm corals, with something else. But are these other chemicals really safer for reefs than oxybenzone?
This question led us, two environmental chemists, to partner with biologists who study sea anemones as a model for corals. Our goal was to find out how sunscreens harm reefs in order to better understand which components of sunscreens are actually “coral safe”.
In our new study, published in Science, we found that when corals and sea anemones take in oxybenzone, their cells convert it into phototoxins, molecules that are harmless in the dark but turn toxic in sunlight.
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Protect people, damage reefs
Sunlight is made up of many different wavelengths of light. Longer wavelengths, like visible light, are generally harmless. But light in shorter wavelengths, such as ultraviolet light, can pass through the skin’s surface and damage DNA and cells. Sunscreens, including oxybenzone, work by absorbing most of the ultraviolet light and converting it to heat.
Coral reefs around the world have suffered in recent decades from ocean warming and other stressors. Some scientists believed that sunscreen from swimmers or sewage discharge could also harm corals. They conducted laboratory experiments showing that concentrations of oxybenzone as low as 0.14 mg per liter of seawater can kill 50% of coral larvae in less than 24 hours.
While most field samples tend to have lower concentrations of sunscreen, a popular snorkeling reef in the US Virgin Islands contained up to 1.4 mg of oxybenzone per liter of seawater. , more than 10 times the lethal dose for coral larvae.
Likely inspired by this research and a host of other studies showing harm to marine life, Hawaiian lawmakers voted in 2018 to ban oxybenzone and another ingredient in sunscreens. Soon after, lawmakers in other coral reef locations, such as the Virgin Islands, Palau, and Aruba, implemented their own bans.
There is still an ongoing debate about whether environmental concentrations of oxybenzone are high enough to harm reefs. But everyone agrees that these chemicals can cause harm under certain conditions, so it’s important to understand their mechanism.
sunscreen or toxin
While lab evidence has shown that sunscreen can harm corals, too little research has been done to understand how. Some studies have suggested that oxybenzone mimics hormones, disrupting reproduction and development. But another theory that our team found particularly intriguing was the possibility that sunscreen behaves like a light-activated toxin in corals.
To test this, we used the sea anemones our colleagues farm as a model for corals. Sea anemones and corals are closely related and share many biological processes, including a symbiotic relationship with the algae that live there. It is extremely difficult to experiment with corals under laboratory conditions, so anemones are generally much better for laboratory studies like ours.
We placed 21 anemones in test tubes filled with seawater under a light bulb that emits the full spectrum of sunlight. We covered five of the anemones with an acrylic box that blocks the exact wavelengths of ultraviolet light that oxybenzone normally absorbs and interacts with. We then exposed all anemones to 2 mg of oxybenzone per liter of seawater.
The anemones under the acrylic box were our “dark” samples and those outside our “light” control samples. Anemones, like corals, have a translucent surface, so if oxybenzone acted as a phototoxin, ultraviolet rays hitting the light group would trigger a chemical reaction and kill the animals, while the dark group would survive.
We carried out the experiment for 21 days. On the sixth day, the first anemone in the light group died. On the 17th, they were all dead. In comparison, none of the five anemones in the dark group died during the three weeks.
Metabolism converts oxybenzone into phototoxins
We were surprised that sunscreen behaves like a phototoxin inside anemones. We performed a chemical experiment with Oxybenzone and confirmed that on its own it behaves like a sunscreen and not a phototoxin. It wasn’t until the anemones absorbed the chemical that it became dangerous in the light.
Whenever an organism takes in a foreign substance, its cells try to get rid of the substance through various metabolic processes. Our experiments suggested that one of these processes converts oxybenzone into a phototoxin.
To test this, we looked at the chemicals that formed inside the anemones after exposing them to oxybenzone. We learned that our anemones had replaced part of the chemical structure of oxybenzone, a specific hydrogen atom in an alcohol group, with a sugar. Replacing hydrogen atoms in alcohol groups with sugars is something plants and animals commonly do to make chemicals less toxic and more water soluble so they are easier to excrete.
But when you remove that alcohol group from oxybenzone, oxybenzone stops working as a sunscreen. Instead, it latches on to the energy it absorbs from ultraviolet light and sets off a series of rapid chemical reactions that damage cells. Instead of converting sunscreen into a harmless, easily excreted molecule, anemones convert oxybenzone into a potent toxin activated by sunlight.
When we conducted similar experiments with mushroom corals, we found something surprising. Although the corals were much more vulnerable to stressors than sea anemones, they did not die from oxybenzone and light exposure during our entire eight-day experiment.
The coral produced the same phototoxins from oxybenzone, but all of the toxins were stored in the symbiotic algae that lived on the coral. The algae appeared to absorb the phototoxic byproducts, and in doing so, they likely protected their coral hosts.
We suspect the corals would have been killed by the phototoxins had they not had their algae. It is not possible to keep living corals without algae in the laboratory, so we did some experiments with anemones without algae. These anemones died about twice as fast and had almost three times more phototoxins in their cells than the same anemones with algae.
Coral bleaching, “reef-safe” sunscreens and human safety
We believe that our efforts to better understand how oxybenzone harms corals are having significant payoffs.
First, coral bleaching events, in which corals expel their symbiotic algae due to high seawater temperatures or other stressors, likely leave corals particularly vulnerable to the toxic effects of sunscreens.
Second, it is possible that oxybenzone is also dangerous for other species. In our study, we found that human cells can also convert oxybenzone into a potential phototoxin. If it happens inside the body, where the light does not reach, there is no problem. But if it happens on the skin, where light can create toxins, it could be a problem. Previous studies have suggested that oxybenzone may pose health risks to people, and some researchers have recently called for more research into its safety.
Finally, the chemicals used in many “reef safe” alternative sunscreens contain the same alcohol group as oxybenzone, so they could also become phototoxins.
Together, we hope our findings will lead to safer sunscreens and help inform reef protection efforts.
This article was written by Djordje Vuckovic, PhD candidate in civil and environmental engineering at Stanford University, and Bill Mitch, professor of civil and environmental engineering at Stanford. It is republished from The Conversation under a Creative Commons license. Articles in English