We’ve known for some time that a commonly used substance in sunscreens affects corals. But now we also understand why, paving the way for coral-safe alternatives.

You may have heard of it: oxybenzone. This substance is often used in sunscreen creams because it acts as a UV filter. However, it has been known for some time that oxybenzone harms already fragile corals. However, the way oxybenzone affects corals has long been a mystery. Until now. Because in a new study, published in the journal scienceresearchers have unraveled the puzzling mechanisms by which the harmful compound hastened the demise of reefs.

Oxybenzone

As early as 2015, scientists came up with worrying news. Sunscreens that save many lives every year because they reduce the risk of skin cancer, among other things, appear to kill the coral at the same time. “The most influential study followed a year later,” said researcher Djordje Vuckovic in an interview with Scientias.nl† “In that study, scientists exposed coral cells and larvae to oxybenzone in a lab. They found that concentrations as low as 0.14 mg per liter killed 50 percent of coral larvae in less than 24 hours.”

enigmatic

It’s bad news. Because every year, a sloppy 6,000 tons of sunscreen – more than the weight of 50 blue whales – washes through American coral reefs. Tourists in particular have been found to carry these large amounts of sunscreen to the reefs. “It would be a sad irony if ecotourism aimed at protecting coral reefs turns out to be exacerbating their decline,” Vuckovic said. Since it is known that the substance oxybenzone harms corals, it has been banned in the US Virgin Islands, Hawaii, the island nation of Palau and the Dutch Bonaire. However, this is not the end of it. Because the exact way in which the harmful substance affects corals was unknown until now. And that makes it difficult to develop sunscreens that are safe for corals.

Study

In the new study, the researchers decided to look for answers. They used anemones as surrogates for corals and then exposed them to oxybenzone in artificial seawater under simulated sunshine. All anemones died within 17 days. However, the anemones exposed to oxybenzone in the absence of simulated sunlight remained viable. This seems to indicate that the presence of oxybenzone causes sunlight to become toxic to corals – the opposite of what it should do.

That’s how it is

“Outside corals, oxybenzone protects against the sun,” explains Vuckovic. “It absorbs harmful UV rays and converts energy into heat. When organisms ingest a chemical compound, their cells try to get rid of it through various metabolic processes. We found that our sea anemones do the same with oxybenzone, replacing part of it with a sugar. Plants and animals often do this to make it easier to excrete chemicals. However, we found that in our case this conversion resulted in a phototoxin – a molecule that is harmless in the dark, but can seriously damage cells and tissue under sunlight.”

bleaching

The further bleaching of corals can also exacerbate this process. Coral lives in symbiosis with microscopic algae that provide the coral with food and color. However, these algae also protect their hosts by storing within themselves the toxins that the corals produce from oxybenzone. But these days, corals are increasingly experiencing stress (due to higher water temperatures and pollution). This causes the algae to leave the tissue of the coral. And that makes corals even more vulnerable; not only for diseases, but also for sunscreen with oxybenzone. “We found that sea anemones die about three times faster without their algae,” says Vuckovic. “In addition, they appear to contain about 2.5 times more phytotoxins.”

Other Concerning Ingredients

Oxybenzone may not even be the only worrisome ingredient in sunscreen, the researchers warn. “Several other ingredients in sunscreen have a chemical structure similar to oxybenzone,” explains Vuckovic. “And they may also be able to form similar phytotoxins.” Whether this is actually the case will have to be further investigated. “We’re actually just now starting to study the other ingredients in sunscreens,” says Vuckovic. “But we think that understanding the mechanisms can help further test and develop sunscreens without chemical structures that can cause damage.”

Alternatives

Vuckovic emphasizes that the findings are based on laboratory research. “There is an open discussion about how well such results can be translated to real reefs,” he says. However, if you want to be on the safe side and after this story you are looking for a sunscreen that does not kill corals anyway, Vuckovic advises the following. “I would personally recommend mineral sunscreens,” he says. In contrast to standard sunscreen with chemical filters, mineral sun protection does not penetrate the skin, but puts a kind of layer on the skin. In addition, they have a fundamentally different effect. The best-known mineral filters are zinc oxide and titanium oxide. “Due to their non-lipophilic nature, I think these sunscreens are less likely to bioaccumulate in corals,” Vuckovic continues. “It means the concentrations may be too low to cause harm, although we haven’t researched the ingredients yet.”

The researchers are taking an important step forward with their study. Because now that we better understand how oxybenzone affects corals, it is easier to search for sunscreens that protect us against the vagaries of the sun in a different, non-harmful way. “My hope is that our research will lead the way towards the development of coral-safe sunscreens,” Vuckovic concludes.