The Toxicity of Tire Wear Particles and Their Leachates on Digestion and Gut Microbiota of Mytilus coruscus

Tire Wear Particles (TWPs) are a major type of microplastics (MPs). However, previous studies have predominantly focused on TWP leachates rather than the particles, and their toxic effects on marine organisms remain limited. In this study, the mussels were exposed to TWPs and their leachates for 21 days, followed by a 7-day recovery period in clean conditions. The results showed that the leachates contained organic pollutants (predominantly PAHs) and metal ions (predominantly Zn2+). Growth inhibition was observed in exposure to TWPs, while leachate exposure showed no significant effect. As for the antioxidant system, high-concentration TWP and their leachate exposures provoked significant oxidative stress, accompanied by inhibited superoxide dismutase (SOD) and catalase (CAT) activity as well as increased glutathione (GSH) and malondialdehyde (MDA) levels. Exposure to high concentrations of TWPs significantly inhibited acetylcholinesterase (AChE) and trypsin (TRS) activities. Gut microbiota analysis indicated that exposure to TWPs and their leachates modified community structure with significantly reduced relative abundance of Firmicutes, Bacteroidota, potentially attributed to the bacteriostatic activity of released Zn2+. Integrated Biomarker Response (IBR) analysis indicated that TWPs elicited stronger integrated toxicity compared to their leachates. This study provides a comparative perspective on the ecological toxicology of TWPs and their leachates.