Soil Heterogeneity Influences the Biotoxicity of Photoaged Tire Wear Particles in Eisenia fetida: A Comparative Assessment

Abstract

Tire wear particles (TWPs) pose escalating environmental concerns owing to their aging-enhanced toxicity. However, the combined effects of photoaging and soil heterogeneity remain poorly understood. This study investigated the impact of virgin and photoaged TWPs on Eisenia fetida in three soil types (red, black, and loess), focusing on earthworm survival, weight loss, antioxidant responses, histopathological changes, and gut microbiota shifts. Results showed that photoaging significantly increased TWP toxicity, as evidenced by elevated surface environmentally persistent free radicals. High-dose exposure to photoaged TWPs caused the greatest reduction in earthworm survival (52.2% vs. control, P < 0.05) and the highest weight loss (0.113 g/worm, P < 0.05) in red soil, whereas the least toxicity (35.6% survival reduction vs. control, P > 0.05) was observed in black soil. The activities of antioxidant enzymes and reactive oxygen species levels were significantly higher in red and loess soils, indicating soil-dependent oxidative stress response. Histopathological analysis revealed tissue damage across all soil types, particularly with photoaged TWPs. Gut microbiota analysis revealed that earthworms in red soil exhibited the least intestinal microbial community shift with minimal diversity/structural changes. TWP exposure specifically increased inflammation-associated Proteobacteria and enriched pathogenic fungal genera. The fungal community response, which was positively correlated with earthworm weight loss (Mantel test, P < 0.05), could serve as a potential biomarker for TWP toxicity. These findings underscore the need for soil-specific risk assessments of TWPs in environmental management.