Insights into microplastic exposure routes in the earthworm (Eisenia fetida): Gut and skin

The uptake of microplastics (MPs) by terrestrial invertebrates poses potential threats to soil ecosystems. This study investigated the uptake of polypropylene MPs in Eisenia fetida, focusing on two exposure routes (gut and skin) and key influencing factors (particle size and concentration). A 50-day uptake-elimination experiment was conducted using four MP concentrations (0.02 %-1.0 %) and two particle size treatments (Normal Size (NS): 31.67-488.02 µm; Small Size (SS): 23.15-232.60 µm). A three-compartment model, gut dissection, and a sealed mouthpart assay assessed uptake routes. Results based on abundance (item·mg⁻¹) indicated MP accumulation positively correlated with concentration, while lower levels enhanced uptake efficiency. SS MPs showed slightly higher accumulation efficiency, with BAFs 1.02-1.49 times greater than NS MPs. MPs in earthworms underwent fragmentation and shape transformation. Modeling and experimental results identified the gut as the primary uptake route, while the skin may serve as a potential excretion route. SS MPs also caused more severe damage to gut and skin tissues, facilitating MP penetration. Additionally, earthworms exposed to 1.0 % MPs exhibited significant uptake of the additive Decabromodiphenyl ethane (DBDPE), with BAFs of 2.71 and 3.04, suggesting bioaccumulation risk. Earthworm activity may further reduce MP size and promote film-like transformations. Overall, MP accumulation in earthworms is influenced by exposure level and particle size, with gut and skin both contributing to uptake and elimination. However, this study could not fully exclude residual MPs in the gut and skin. Future studies should employ refined anatomical techniques to better delineate exposure routes and tissue accumulation.