Soil Matrix Modulates Polystyrene Microplastic Toxicity to Eisenia fetida: From Acute Lethality to Oxidative Genotoxicity

Microplastic (MP) contamination in soils has raised widespread concern, yet the extent to which soil matrices regulate MP toxicity remains insufficiently characterized. In this study, the acute and subchronic toxicity of polystyrene (PS) to the earthworm (Eisenia fetida) was analyzed in artificial soil and three types of natural soils (lou soil, black soil and latosol soil). The 28 d LC50 (50% lethal concentration) of PS in artificial soil was 130.86 g kg−1, whereas it decreased to 39.71 and 39.23 g kg−1 in lou and latosol soil, respectively, and increased to 175.13 g kg−1 in black soil, indicating that soil physicochemical properties may modulate MP toxic potency. Across all soil types, PS exposure triggered reactive oxygen species (ROS) accumulation in earthworms, which in turn activated antioxidant and detoxification enzymes (superoxide dismutase, catalase, and glutathione S-transferase) and ultimately led to lipid peroxidation and DNA damage. Assessment via the integrated biomarker response index suggested divergent PS toxicity across soils, indicating inconsistencies between artificial soil and natural soils. Notably, this study was conducted under controlled laboratory conditions using a single polymer type, particle size, and test organism, and therefore the observed soil-dependent toxicity patterns may not be directly extrapolated to other MP types, sizes, or soil biota. Overall, our study suggested that the toxicity data derived from artificial soil cannot fully represent the real effects in natural soils, providing valuable insights for elucidating the realistic toxicity of MPs.