Ecotoxicology of inorganic sunscreen on tropical corals in a warming ocean

Abstract

Titanium dioxide nanoparticles (nTiO2) are popular UV filter ingredients incorporated in inorganic sunscreen formulations to effectively protect human skin from the harmful effects of sun rays. Because of their extensive use, in recent years sunscreen products have emerged as potential contaminants of environmental concern, especially in coastal waters where sunscreen ingredients are washed off from the skin of bathers and discharged by sewage following showering and laundry. Various studies have documented the toxicity of organic UV filters to corals, however very little is known about the impacts of nTiO2 UV filters and whole sunscreen formulations. Moreover, their potential interaction with ocean warming is unknown. The aim of this project was therefore the characterization of the photo-physiological and reproductive performances of tropical corals exposed to inorganic sunscreen ingredients and formulations, both as individual stressors and combined with elevated temperature conditions projected for the next decades. First, the potential toxicity of different nTiO2 UV filters to cultured coral’s endosymbiotic algae (Symbiodiniaceae) was investigated. Second, laboratory work was performed to assess the effects of a typical nTiO2-based sunscreen formulation on the metabolism and photosynthetic activity of two coral species. The sea anemone Exaiptasia pallida was then used as a model organism to compare the toxicity of sunscreen formulations with and without UV filter. E. pallida experimentation allowed also the characterization of early gene expression of the stress-related heat-shock proteins (HSPs) at the onset of sunscreen-derived stress. Finally, fieldwork was conducted to evaluate the toxicity of inorganic sunscreens on corals’ early life history stages, and to determine whether sunscreen toxicity change in relation to the emulsifying agents in the formulation. Findings from this study highlight the importance of considering whole formulations, and especially the emulsifier ingredients, in evaluating sunscreen toxicity. Exposure to inorganic sunscreen’s estimated environmental concentration induced significant harmful effects on reef-building corals. Symbiodiniaceae growth was reduced, coral metabolism and photosynthetic activity declined and HSPs gene expression was highly upregulated from the onset of sunscreen exposure. Inorganic sunscreen exposure also caused an increase in embryo abnormalities and a reduction of sperm motility and larvae survival. These effects were worsened under warming scenarios, suggesting that corals living in sunscreen-contaminated waters may experience a reduced resistance to thermal stress.