Ecotoxicological effects of polyethylene microplastics and ZnO nanoparticles on earthworm Eisenia fetida

Abstract

With the increasing release of microplastics (MPs) and nanoparticles (NPs) into the environment, there has been a growing concern about the co-occurrence of these emerging contaminants. However, little is known about the co-contamination effects of MPs and NPs on terrestrial organisms. For the first time, we studied the individual and combined effects of polyethylene (PE) MPs (0, 1%, and 10%, w/w) and ZnO NPs (0, 100, 500, and 1000 mg/kg) on earthworms (Eisenia fetida). Compared to the control, PE MPs or ZnO NPs alone increased the weight loss rate and mortality of earthworms, while the combined exposure of PE MPs and ZnO NPs resulted in higher weight loss rate at the 28th day and lower mortality compared to single application. ZnO NPs greatly increased soil available Zn content, with the highest increments by 1000 mg/kg ZnO NPs at the 28th day. Zn content in earthworms was not affected after 14 days of exposure to ZnO NPs, but showed an increasing trend with increasing ZnO NPs after 28 days of exposure. Co-occurring MPs did not affect soil available Zn content, but changed Zn accumulation in earthworms after 28 days of exposure. The highest Zn content (55.6 mg/kg) was observed in the earthworms under the co-exposure to 1% PE and 1000 mg/kg ZnO NPs. Single exposure to PE MPs or ZnO NPs caused oxidative stress and histopathological damage in the epidermis, intestinal tracts and seminal vesicles. However, co-exposure caused higher CAT and GST activity and MDA content, and more severe histopathological damage, indicating a combined toxicity between them. In conclusion, co-exposure to PE MPs and ZnO NPs caused higher Zn bioaccumulation and more pronounced toxicity response in earthworms, manifesting greater ecological risks. Our findings advance understanding the ecotoxicological risks of co-contamination of MPs and NPs to soil fauna.