Assessing the Accumulation of Microplastics in Earthworms ( <i>Eisenia fetida</i> ) Using Traditional Bioaccumulation Modeling and Synchrotron-Based Microcomputed Tomography

Microplastics (MPs) are recognized as a ubiquitous contaminant in terrestrial ecosystems. Earthworms, which play key roles in soil structure, nutrient cycling, and ecosystem function, are important bioindicators for assessing MP exposure and fate. However, uncertainties remain regarding MP toxicokinetics in earthworms, including ingestion dynamics, retention, and the potential for translocation across the gut epithelium. This study investigated the movement and fate of MPs in Eisenia fetida by quantifying uptake and elimination across five common MP morphologies (films, fibres, foams, fragments, and spheres) and by using synchrotron-based microcomputed tomography (SR-µCT) to visualize internal distributions of 5 to 22 µm glass microspheres and 45 to 53 µm polyethylene microspheres. Across 21-day exposure and depuration phases, earthworms rapidly ingested all MP morphologies, reaching steady state before Day 10 for most types, and efficiently eliminated MPs once removed from spiked soils. Elimination rate constants (1.27-2.34 day-1) corresponded to short half-lives (<1 day), and bioaccumulation factors (BAFs) were consistently low (0.023-0.058), indicating limited retention relative to soil concentrations. SR-µCT imaging provided high-resolution three-dimensional confirmation that MPs remained entirely confined to the gastrointestinal tract, with none of the 2,779 detected particles ranging from 5 to 53 µm crossing the gut epithelium. Despite higher MP densities in earthworms exposed to smaller glass microspheres, this pattern reflected differences in ingestion rather than tissue penetration. Reduced gut-to-worm volume ratios in polyethylene-exposed worms suggested decreased feeding or avoidance behaviour toward the polyethylene microspheres. Overall, these results demonstrate that Eisenia fetida rapidly ingest and eliminate MPs without tissue translocation, and that their gut contents provide a reliable short-term snapshot of MP availability in soil.