096 | Hidden threats: trojan horse effects of microplastics enhance the toxicity of tire-wear chemicals in marine ecosystems

Federica Davì, Roberta Fusco, Davide Di Paola | Department of Chemical, Biological, Pharmaceutical, and Environmental Sciences, University of Messina, Messina, Italy.

Tire wear particles (TWPs) are an increasingly recognized source of microplastics and associated chemical additives in aquatic environments. Among these, 6-PPD-quinone (6-PPD-Q), a transformation product of the tire antioxidant 6PPD, has emerged as a contaminant of global concern due to its acute toxicity to freshwater fish. However, the effects of 6-PPD-Q in marine environments, particularly under coexposure with microplastics (MPs), remain poorly understood. This study investigated the combined effects of 6-PPD-Q and MPs in the Mediterranean mussel Mytilus galloprovincialis, a key marine bioindicator. Using an integrated multilevel approach, we assessed molecular, biochemical, metabolic, and histopathological responses following single and combined exposures. While individual stressors induced moderate antioxidant activation, co-exposure resulted in pronounced oxidative exhaustion characterized by the depletion of total antioxidant capacity, non-linear modulation of antioxidant gene expression, and elevated superoxide dismutase and glutathione peroxidase activities. These effects were accompanied by increased energetic demand, as indicated by enhanced electron transport system activity and upregulation of detoxification pathways, suggesting a costly redistribution of metabolic resources. Despite these compensatory responses, co-exposed mussels exhibited strong activation of pro-apoptotic signaling and severe structural damage in gills and digestive glands, indicating a collapse of cellular homeostasis. Multivariate analysis confirmed that the interaction between 6-PPD-Q and MPs generates a distinct and more severe toxicological profile than either stressor alone. Our findings support a ‘Trojan Horse’ mechanism whereby microplastics exacerbate 6-PPD-Q toxicity by enhancing internal exposure. Overall, this study demonstrates that synergistic effects between tire-derived chemicals and microplastics substantially amplify ecological risk, highlighting the need for mixture-aware approaches in marine environmental risk assessment.