Microplastic ingestion induces changes in coelomocyte composition of Eisenia fetida

The earthworm Eisenia fetida is a widely used model organism for environmental toxicity assessment, including microplastic particles (MPP). Impacts of MPP at the organismic level have been extensively documented. Effects on the cellular level and in particular the immune system of E. fetida are less well understood. Here we exposed earthworms outside soil for six days to food pellets containing 10% (w/w) pure polystyrene (PS) MPP (25 – 75 µm) or PS MPP supplemented with 0.5% (w/w) of the antioxidant Irgafos (PSIrgafos). Immune cells (coelomocytes) were isolated from E. fetida using a non-invasive extrusion method, and their number, viability, and subpopulation distribution (amoebocytes and eleocytes) were assessed by flow cytometry. Husbandry outside soil was found to generally increase propensity for apoptosis in particular of amoebocytes with little difference between experimental setups. The total number of extruded cells was not affected by MPP, while coelomocyte viability was significantly reduced. PSIrgafos ingestion further decreased the number of living cells per mg fresh weight. Ex vivo analysis of coelomocyte viability confirmed an enhanced toxicity of PSIrgafos compared to PS. Our findings show that MPP and their additives cause physiological changes and alter immune cell distribution, posing potential risks to soil invertebrates.