Divergent impacts of leached versus pristine tire wear particles on zebrafish: From physiological stress to gut microbiota dysbiosis

Extensive research has been conducted on the leaching behavior and risks of the leachate of tire wear particles (TWPs) in aquatic environments. However, whether the toxicity of TWPs is altered following the leaching process remains largely unexplored. This study systematically examined the ingestion, bioaccumulation, histopathological damage, oxidative stress, gene expression, and intestinal microbial responses in zebrafish exposed to TWPs pretreated by ultrapure water or methanol leaching. A clear toxicity gradient was identified: original TWPs (TWP-Ori) caused mild damage, water-leached TWPs (TWP-UP-60) caused moderate damage, and methanol-leached TWPs (TWP-Me-60) induced the most severe and persistent tissue injury in the gill, gut, and liver. TWP-Ori was rapidly ingested and caused acute chemical toxicity, including hepatic oxidative stress, inflammation, and marked upregulation of metal detoxification genes. In comparison, TWP-UP-60 and TWP-Me-60 showed reduced palatability and altered bioaccumulation, with sporadic ingestion and rapid elimination, respectively. All leached particles severely disrupted the gut ecosystem, sharply reducing microbial diversity (Shannon index from 5.49 to 2.20) and shifting community structure. TWP-Ori enriched anaerobic stress-tolerant Bacillota, whereas leached TWPs led to overwhelming dominance of Pseudomonadota (77.1–91.5%). These results demonstrate that leaching shifts the primary hazard of TWPs from acute chemical toxicity to persistent chronic disturbance, characterized by tissue damage, oxidative stress, and gut microbial dysbiosis.