Photodegradation of Plastic Leachate: Revealing the Key Role of Halogen in Reduced Cytotoxicity in Marine Systems

Understanding the cytotoxicity of plastic leachate from photodegraded sunscreen-derived microplastics (SDMPs) in various aquatic environments remains a challenge. This study reveals the crucial role of halogens in reducing leachate toxicity by examining the photodegradation of SDMPs in freshwater and seawater settings. Microplastics (MPs) extracted from three different commercial sunscreens, predominantly composed of methacrylate-based polymers, exhibited reduced photodegradation, leachate release, and leachate transformation in seawater. The diminished degradation in seawater can be attributed to halogens, especially bromide (Br^-), which hinder photocatalytic oxidation by suppressing the activity of hydroxyl radicals (^•OH). Consequently, leachates from SDMPs photoaged in seawater exhibited notably reduced cytotoxicity, as evidenced by minimal mitochondrial dysfunction involving mitochondrial dehydrogenases and membrane potential, fragmented mitochondria, and regulated metabolic processes. This included the alleviation of amino acid catabolism inhibition, coenzyme A (CoA) biosynthesis inhibition, glycolysis activation, and the enhancement of the compensatory antioxidant system. In the absence of halogen protection, photodegradation is predicted to produce oxidative byproducts such as hydroperoxides as toxic agents that induce cytotoxicity in the freshwater leachate. These findings underscore the critical influence of water chemistry on shaping the photodegradation and toxicity of SDMP leachates, emphasizing the necessity of conducting region-specific risk assessments for MPs in personal care products.