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.