Oxybenzone in sunscreens make sunlight toxic for corals
Vince Erick U. Reyes
“Oxybenzone is good at absorbing light within the waveband we tested, but it was strange to see that it made sunlight toxic for corals; the opposite of what it is supposed to do” — Mitch (2022)
With climate change on the rise, various ecosystems continue to receive detrimental impacts as a result of anthropogenic activities; included here is the problem of sunscreen pollution, which contributes immense damage to the coral reefs that serve as habitats to a wide variety of marine species. Worn by tourists who explore reef attractions for purposes of snorkeling or diving, about 4,000–6,000 tons of sunscreen wash off in U.S. reef areas each year.
While the adequate use of sunscreen is a protective measure for humans, compounds contained in such products contrastingly cause harmful effects to corals. Oxybenzone, a common component of many sunscreens, is excellent at absorbing ultraviolet (UV) light and dissipating it as heat. However, it has long been known for its adverse environmental effects which caused Hawaii and other jurisdictions to ban the use of oxybenzone-containing sunscreens; although scientists at that time were yet to know the specific mechanism by which it causes corals to die.
The ambiguity was resolved when a group of Stanford University researchers led by Djordje Vuckovic, a PhD student in civil and environmental engineering, discovered a possible mechanism to explain such a phenomenon. It was found in their study that when sunscreens run off in coral reefs, the oxybenzone does not simply dissipate as heat and is rather metabolized by corals, resulting in the production of damaging radicals particularly in the presence of sunlight.
The team observed the phenomenon among glass anemones (Exaiptasia pallida) by simulating the conditions through artificial setups of seawater and sunlight. Specifically, the glass anemones in the controls (either exposed to oxybenzone alone or UV light alone) remained viable throughout the 21-day experimental period. On the other hand, the anemones exposed to both oxybenzone and UV light died within 17 days. It was found that in the presence of sunlight, sugars were added to oxybenzone and the produced oxybenzone-sugar compound was activated as a toxin, contrary to the typical function of sugar add-ons on the detoxification of chemicals. The presence of this toxin is directly responsible for the death of the corals.
In addition to this discovery, symbiotic algae were also determined to play a vital role in the corals’ defense mechanism. Particularly, it was observed that the algae living inside mushroom corals (Discosoma sp.) protected its hosts by the sequestration and absorption of the said toxin. None of the corals died after a combined exposure to UV light and oxybenzone within an 8-day period.
Meanwhile, corals without innate algae but can form symbiotic relationships e.g. glass anemones were also considerably safeguarded from the damaging effects of the toxin. As simulated, anemones which lacked algae died quickly, a few days sooner than the anemones with symbiotic algae residing on its skeleton. Due to drastic changes in environmental conditions brought about by climate change, this observation translates to bleached or heat-stressed corals being more vulnerable to death due to the lack of algae that can protect it from the toxin.
With this research, the team is hoping that their findings will be of substantial contribution to the development of coral-safe sunscreens. Further information on the work can be found in their paper published in the May 6 issue of the journal Science.
