Corals and sea anemones turn sunscreen into toxins – understanding how could help save coral reefs

A new study found that when corals and sea anemones absorb oxybenzone, their cells turn it into phototoxins. What does that mean?

From The Conversation

Sunscreen bottles are frequently labelled as “reef-friendly” and “coral-safe”. These claims generally mean that the lotions 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 team up with biologists who study sea anemones as a model for corals. Our goal was to uncover how sunscreen harms reefs so that we could better understand which components in sunscreens are really “coral-safe”.

In our new study, published in Science, we found that when corals and sea anemones absorb oxybenzone, their cells turn it into phototoxins, molecules that are harmless in the dark but become toxic under sunlight.

Protecting people, harming reefs

Sunlight is made of many different wavelengths of light. Longer wavelengths – like visible light – are typically harmless. But light at shorter wavelengths – like ultraviolet light – can pass through the surface of skin and damage DNA and cells.

Sunscreens, including oxybenzone, work by absorbing most of the UV light and converting it into heat.

Coral reefs around the world have suffered in recent decades from warming oceans and other stressors. Some scientists thought that sunscreens coming off of swimmers or from waste-water discharges could also be harming corals.

They conducted lab experiments that showed that oxybenzone concentrations as low as 0.14 mg per litre of seawater can kill 50 per cent of coral larvae in less than 24 hours.

While most field samples typically have lower sunscreen concentrations, one popular snorkeling reef in the US Virgin Islands had up to 1.4 mg oxybenzone per litre of seawater -- more than 10 times the lethal dose for coral larvae.

Likely inspired by this research and a number of other studies showing damage to marine life, Hawaii’s legislators voted in 2018 to ban oxybenzone and another ingredient in sunscreens.

Soon after, lawmakers in other places with coral reefs, like the Virgin Islands, Palau and Aruba, implemented their own bans.

There is still an open debate whether the concentrations of oxybenzone in the environment are high enough to damage reefs. But everyone agrees that these chemicals can cause harm under certain conditions, so understanding their mechanism is important.

Sunscreen or toxin

While laboratory evidence had shown that sunscreen can harm corals, very little research had been done to understand how. Some studies suggested that oxybenzone mimics hormones, disrupting reproduction and development. But another theory that our team found particularly intriguing was the possibility that the sunscreen behaved as a light-activated toxin in corals.

To test this, we used the sea anemones our colleagues breed as a model for corals. Sea anemones and corals are closely related and share a lot of biological processes, including a symbiotic relationship with algae that live within them.

It is extremely difficult to perform experiments with corals under lab conditions, so anemones are typically much better for lab-based studies like ours.

We put 21 anemones in test tubes full of seawater under a lightbulb that emits the full spectrum of sunlight. We covered five of the anemones with a box made of acrylic that blocks the exact wavelengths of UV light that oxybenzone normally absorbs and interacts with. Then we exposed all the anemones to 2 mg of oxybenzone per litre of seawater.

The anemones under the acrylic box were our “dark” samples and the ones outside of it our control “light” samples.

Anemones, like corals, have a translucent surface, so if oxybenzone were acting as a phototoxin, the UV rays hitting the light group would trigger a chemical reaction and kill the animals – while the dark group would survive.

We ran the experiment for 21 days. On day six, the first anemone in the light group died. By day 17, all of them had died. By comparison, none of the five anemones in the dark group died during the entire three weeks.

Metabolism converts oxybenzone to phototoxins

We were surprised that a sunscreen was behaving as a phototoxin inside the anemones. We ran a chemical experiment on oxybenzone and confirmed that, on its own, it behaves as a sunscreen and not as a phototoxin.

It’s only when the chemical was absorbed by anemones that it became dangerous under light.

Any time an organism absorbs a foreign substance, its cells try to get rid of the substance using various metabolic processes. Our experiments suggested that one of these processes was turning oxybenzone into a phototoxin.