Bioavailability of pyrene in soil affected by polylactic acid and polystyrene microplastics and their toxic effects on earthworms (Eisenia fetida)

Microplastics (MPs) can adsorb numerous pollutants, thus affecting the bioavailability and toxicity of pollutants. The toxic effects and bioavailability of pyrene (Pyr) in soils containing polylactic acid (PLA) and polystyrene (PS) were analysed using earthworms (Eisenia fetida). The results showed that PS and PLA had different effects on the pyrene bioavailability of soil. The mortality, weight loss rate and 8-hydroxy-2'-deoxyguanosine contents of earthworms did not significantly change; however, antioxidant enzymes and acetylcholine esterase activities changed to varying degrees, while malondialdehyde content increased first and then decreased. This trend indicates that MPs did not lead to the breakdown of the antioxidant system but rather caused neurotoxicity. Pyrene (5 mg/kg) treatment caused significant oxidative stress and neurotoxicity (p < 0.05). Combined exposure to MPs and Pyr induced more complex and severe toxic effects compared to Pyr alone. The Alpha and Beta diversity of earthworm gut microbiota were affected by both Pyr and the combined exposure to MPs and Pyr; the type of MP affected the microbiota structure (p < 0.01). These changes proved that pyrene alone and the combined treatment of MPs and Pyr caused more serious toxicity to earthworm gut microbiota than MPs alone. Overall, this study provides important insights into the single and combined toxicity of MPs and Pyr.