According to the National Park Service, up to 6,000 tonnes of sunscreen equivalent to the weight of 50 blue whales wash across U.S. reef regions every year. For a long time, scientists have known that oxybenzone, an organic chemical included in many sunscreens, might harm corals. As a result, sunscreens containing this chemical have been banned in the US Virgin Islands and Hawaii, as well as the island nation of Palau and Bonaire, a Dutch island municipality. However, the mechanisms by which oxybenzone harms corals are largely unknown, making it difficult to ensure that sunscreen components proposed as alternatives are truly safer for corals.
MECHANISM
"The compound is good at absorbing light , which is why it's so common in sunscreens."
Oxybenzone works by absorbing UV light, then releasing the energy as heat. This is due to presence of the OH group on oxybenzone. Once in the high energy state, the OH group is able to wiggle the energy away. But this is no longer possible once the corals metabolize oxybenzone. The compound reacts via a glycosylation reaction, where the OH is deprotonated and a glucose molecule adds to the O-. The new oxybenzone glucoside conjugate can still absorb light, but now does not have a way to release the energy as heat. Instead, the excited compound forms reactive oxygen species. “That sets off this radical chain reaction, where eventually it ends up causing damage to cells or tissue.
In addition to this vulnerability, the researchers found evidence for a coral defense mechanism. Symbiotic algae in corals appeared to protect their hosts by sequestering within themselves the toxins that corals produced from oxybenzone.
As ocean waters warm, stressed corals expel their algae partners, exposing bone-white coral skeletons. Thus, in addition to being more vulnerable to disease and environmental shocks, such "bleached" corals would be more vulnerable to the depredations of oxybenzone without their algae to protect them.
"Oxybenzone makes sunlight toxic for corals". The corals basically convert oxybenzone, a sunscreen, into what is essentially the opposite of a sunscreen, a phototoxin.
Ensuring that all sunscreens are not harmful to corals and other marine life.
According to the researchers, oxybenzone isn't the only sunscreen chemical to be concerned about. The same metabolic mechanisms that appear to turn oxybenzone into a strong coral toxin may also convert other common sunscreen chemicals into phototoxic metabolites, as many of them have similar chemical structures and so potentially generate similar phototoxic metabolites.
Many coral-safe sunscreens are made with metals like zinc and titanium rather than chemical components like oxybenzone. Despite the fact that these sunscreens work in fundamentally different ways, the researchers say it's unclear whether they're genuinely safer for corals. They aim to look into it further.
Source: Stanford University
Summary: Researchers have discovered how oxybenzone, a common sunscreen ingredient, harms corals. The unexpected discoveries could aid in the development and marketing of coral-safe sunscreens.
Journal Reference: Djordje Vuckovic, Amanda I. Tinoco, Lorraine Ling, Christian Renicke, John R. Pringle, William A. Mitch. Conversion of oxybenzone sunscreen to phototoxic glucoside conjugates by sea anemones and corals. Science, 2022; 376 (6593): 644
Very informative! 👏