Microplastic cytotoxicity and the phagocytic response of earthworm immune cells

Agricultural soil is a major sink for microplastics, which accumulate as a consequence of plastic mulching, wastewater irrigation, and the application of organic materials containing plastic residues as soil amendments. Soil organisms like earthworms are sensitive to microplastic exposure. However, a global regulatory gap currently exists regarding small plastic fragments (< 2 mm) in agricultural soils, and there is limited information on the influence of size-dependent microplastics on the direct cellular immune responses of earthworms, particularly under in vitro conditions. In this paper, we conducted an in vitro investigation of the cytotoxicity of polyethylene microplastics in Eisenia fetida coelomocytes, which revealed size-dependent differences in phagocytosis. Coelomocytes engulfed 85% of small polyethylene microplastics (1-10 µm) through phagocytosis, but there was negligible phagocytosis of larger polyethylene microplastics (20-27 µm). Cell viability was 6-7% when coelomocytes were exposed to 1-10 µm and 20-27 µm-sized microplastics, significantly less than the 94% viability of coelomocytes in the untreated control. This finding highlights distinct cytotoxic pathways triggered by different microplastic size: small microplastics (1-10 µm) accumulating in cells, whereas larger microplastics (20-27 µm in this study) appear to cause frustrated phagocytosis, causing inflammation or cellular damage or death.