Ecotoxicological effects of combined exposure to bifenthrin and polyethylene microplastics on the earthworm Eisenia...

Bifenthrin (BF) is a widely used insecticide, and polyethylene microplastics (PE MPs) are emerging contaminants in agricultural soils. However, the combined ecotoxicological effects of BF and PE MPs on soil organisms remain poorly understood. In this study, a controlled soil exposure experiment was conducted in which earthworms (Eisenia fetida) were exposed to BF (10 mg/kg) and/or PE-MPs (30 μm, 2.5 g/kg) for 14 and 28 days. The ecotoxicological effects were evaluated using integrated biochemical, histopathological, and transcriptomic approaches. Molecular dynamics simulations revealed that BF could spontaneously adsorb onto PE surfaces, with an adsorption energy of -30.58 kcal/mol. Specifically, co-exposure to BF and PE-MPs significantly elevated ROS, which was 3.5% higher than in the BF group, and markedly inhibited AChE activity, which was 12.2% lower than in the BF group after 28 days, indicating enhanced oxidative stress and neurotoxicity. Prolonged exposure suppressed key antioxidants, including superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH), indicating a transition from adaptive response to oxidative damage. Integrated biomarker analysis showed the highest IBR under co-exposure, with positive EAI values of 0.213 and 0.191 after 14 and 28 days, respectively, confirming persistent synergistic toxicity. Histopathological observations confirmed aggravated tissue injury, particularly in the intestinal epithelium and muscle layers, under combined exposure. Transcriptomic analysis revealed that co-exposure induced broader transcriptional perturbations involving stress-responsive signaling, membrane-associated dysfunction, and detoxification transport pathways. Overall, PE-MPs amplify BF toxicity through synergistic interactions, highlighting the importance of considering pollutant co-exposure and carrier effects in ecological risk assessment of contaminated soils.