Combined toxicity of microplastics and arsenic to earthworm (Eisenia fetida): a comparison of polyethylene, polylactic acid, and polybutylene adipate-co-terephthalate

Microplastics (MPs) derived from degraded agricultural mulching films frequently co-occur with arsenic in cropland soils, yet their joint toxic effects on soil fauna remain insufficiently understood. This study compared conventional non-biodegradable microplastics (polyethylene, PE) with biodegradable microplastics (polylactic acid, PLA; and polybutylene adipate-co-terephthalate, PBAT) co-exposed with arsenate (As(V)) in a 28-day soil exposure experiment using earthworm Eisenia fetida as model organism. Results showed that microplastic abundance in the earthworm gut increased over time, whereas co-exposure consistently reduced arsenic bioaccumulation in tissues, with stronger reductions observed for PLA and PBAT than for PE. Across multiple physiological and oxidative stress-related biomarkers, As(V) induced growth inhibition and oxidative injury, while the presence of microplastics generally mitigated these adverse responses. Integrated Biomarker Response (IBR) analysis indicated lower overall stress in the combined treatments than in the corresponding single As(V) treatments, and interaction assessment based on the IBR-Effect Addition Index (EAI) model further suggested antagonistic effects between As(V) and all three microplastics. Overall, biodegradable microplastics (PLA MPs and PBAT MPs) exhibited a greater capacity to attenuate arsenic toxicity than conventional PE MPs. These findings demonstrate polymer-dependent interactions in arsenic-microplastic co-contamination and provide a scientific basis for ecological risk assessment of conventional and biodegradable mulching films in agricultural soils.