CAS: Towards sustainable sunscreens: identifying chemical structures in sunscreens linked to phototoxicity in corals

Stanford University, Stanford CA

Investigators

Abstract

A large fraction of the world's coral reefs are at a high risk of extinction. Although much of the threat is due to global factors, coral declines can also be significantly exacerbated by local factors. Recent concern has focused on oxybenzone, a common sunscreen component, due to observations that oxybenzone is toxic to adult corals and reduces survival of juvenile coral. The reasons for this toxicity are unclear, and improved understanding is needed to enable the design of sustainable sunscreens that are truly safe for coral reefs. This research project will characterize the chemical structures within organic sunscreen components or their metabolites that are responsible for phototoxicity to corals, focusing on five of the most common organic sunscreen components: oxybenzone, homosalate, octisalate, octinoxate, and octocrylene. This improved understanding will lay the groundwork for the design of new and sustainable sunscreen components that do not exhibit phototoxicity. The central hypothesis is that the presence or absence of a hydroxyl group adjacent to the aromatic carbonyl (present in each of the five compounds) is a critical determinant of whether a sunscreen component is phototoxic. Sunlight absorption forms excited triplet states of aromatic carbonyls that can generate Reactive Oxygen Species (ROS), resulting in phototoxicity. However, the hydroxyl group adjacent to the aromatic carbonyl in oxybenzone, homosalate, and octisalate inhibits ROS production during sunlight illumination by quenching excited-state triplets. The hypothesis, backed by preliminary evidence, is that oxybenzone, homosalate, and octisalate themselves are not phototoxic, but that coral metabolism glycosylates the hydroxyl groups to form glucoside metabolites that produce ROS and are thus phototoxic. Octinoxate and octocrylene should be directly phototoxic because they lack a hydroxyl group adjacent to the aromatic carbonyl. The research will employ both hard- and soft-coral adults but mainly use the sea anemone Aiptasia, a well characterized model system for corals; like corals, Aiptasia are anthozoans containing endosymbiotic photosynthetic algae. Objective 1 will characterize the mechanism of oxybenzone phototoxicity by: (i) testing whether the presence of oxybenzone leads to mortality only in the presence of the UV wavebands of sunlight, (ii) quantifying the conversion of oxybenzone to glucoside metabolites by the host animal and/or the symbiotic algae, (iii and iv) testing for ROS production under sunlight from glucoside metabolites (iii) in vitro and (iv) in vivo, and (v) quantifying mortality from exposure to glucoside analogues. Objective 2 will replicate these experiments with homosalate, octisalate, octinoxate, and octocrylene to characterize the relevance of this mechanism to sunscreen components being considered as alternatives to oxybenzone. Undergraduate students from underrepresented groups and a high-school science teacher with an accompanying student from an underrepresented group will be recruited to participate in summer research by partnering with existing Stanford University programs. The research should inform policy discussions about which components to regulate in sunscreens and be relevant to the management of coral-reef reserves. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.