Specific toxicity of octinoxate and octocrylene on Symbiodinium sp., a symbiotic microalga with corals

The widespread use of UV filters in sunscreens and personal care products has raised concerns about their detrimental effects to the aquatic environment. This study examined the specific toxicity of two UV filters, octinoxate and octocrylene to Symbiodinium sp., a photosynthetic dinoflagellate essential for coral symbiosis, nutrient acquisition, and reef structure. The study employed a comprehensive set of sub-lethal endpoints analyzed through flow cytometry, including cell viability, pigment fluorescence, cell size, complexity, metabolic activity, production of reactive oxygen species and membrane potential. The exposure of exponentially proliferating Symbiodinium sp. to octinoxate and octocrylene demonstrated pronounced toxicity, with octinoxate exhibiting toxicity levels significantly greater than those of octocrylene. This disparity underscores the different ecological impacts of these UV filters. Even at lower concentrations, octinoxate significantly influenced cellular parameters, including cell size, complexity, viability, and metabolic activity, as evidenced by increased lipid peroxidation (LPO) and neutral lipid accumulation, alongside a reduction in cellulose levels, suggesting potential structural alterations in cellular components. In contrast, octocrylene's sub-lethal effects are manifested as enhanced cell complexity and LPO, with elevated neutral lipids and cellulose levels. However, at elevated concentrations, octocrylene adversely affected cell viability and metabolic activity, indicative of severe membrane depolarization. These findings highlight the importance of an early warning system to protect Symbiodinium sp. and consequently corals. Flow cytometry proved to be a valuable diagnostic tool for detecting sub-lethal effects, providing insights into Symbiodinium sp. health status and, consequently, the resiliency of coral reef ecosystems.