Effects of sunlight exposure on tire tread particle leachates: Chemical composition and toxicity in aquatic systems

Tire tread particles (TTP) are small micro- or nano-particles resulting from the friction of tire tread against roadways. These secondary microplastics have been found in waterways, arriving through airborne means or runoff. Due to their abundance and persistence in aquatic environments, TTP pose a potential hazard to wildlife. Natural degradation processes like photoirradiation can potentially worsen this by transforming leached TTP chemicals. In this study, we assessed the toxicity and chemical composition of TTP leachates produced over 1 or 6 days in either dark or photoirradiated conditions. For toxicity studies, zebrafish embryos were exposed to leachates over a range of concentrations and from 0 to 4 days post fertilization. TTP exposures impaired survival and hatching, induced embryonic defects, and modulated detoxification by the enzyme ethyoxyresorufin-O-deethylase. RNA sequencing revealed divergent effects based on photoirradiation, including impacts on glycolysis, lipid metabolism, and mitochondrial function. For chemical analysis, leachates were assessed using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC/TOF-MS) and chromatographic features were annotated. In total, 546 chromatographic features were detected across all samples, and clustering showed unique chemical profiles based on photoirradiation during leaching. Several compounds were in high abundance in 1-day irradiated leachates, including 1,3-diphenylguanidine, aniline, and 1H-benzotriazole, though their relative abundance was reduced in 6-day leachates. Overall, this research compounds on the existing literature defining TTPs as toxic microplastics in the environment, and we show novel chemical and toxicological data that demonstrates how photoirradiation in the natural environment may exacerbate toxicity.