Aged biodegradable and non-biodegradable microplastics alter metolachlor toxicity in the gut and liver of crucian carp

Degradable plastic mulch films in agriculture, like non-degradable films, can generate microplastics (MPs) and interact with pesticides. Although some studies have examined MP-pesticide combined toxicity, aquatic organism research remains scarce. This study investigated the combined toxicities of metolachlor (MET) at environmentally relevant levels (3 μg/L) and high concentrations (300 μg/L) with aged MPs from non-degradable polyethylene (PE) and degradable polylactic acid (PLA) on crucian carp. The PE significantly increased MET accumulation in the liver and gut compared to MET alone group, whereas PLA exhibited the opposite trend. The MET alone exposure induced gut and liver damage in crucian carp, including oxidative damage, immune inflammatory responses, and metabolic disorders. Co-exposure of 3 μg/L MET with PE induced greater gut toxicity than with PLA, but at 300 μg/L MET, the MET + PLA combination was more gut toxic, primarily due to degradation of PLA. The toxic effects in the MET + PE group had a higher similarity to MET alone exposure. In contrast, MET+PLA groups showed more complex toxic effects and mechanism, which might be caused by the co-action of aged small particle PLA and MET. This study underscores the necessity of considering the ecological risk assessment of biodegradable films in agricultural applications.