Polystyrene micro- and nanoparticles induce ultrastructural alterations and disrupt gametogenesis and midgut homeostasis in the soil oligochaete <i>Enchytraeus albidus</i>

Plastic pollution in terrestrial ecosystems is an increasing concern, yet its impact on soil fauna, particularly minute annelids, remains understudied. This study investigated the effects of polystyrene micro- and nanoparticles (MPs, NPs) on the White worm Enchytraeus albidus, a minute soil-dwelling saprophage that continuously ingests and processes soil, making it highly and directly exposed to terrestrial plastic contamination. This species is an ideal model for soil ecotoxicology due to its ecological role in soil structure modification and nutrient cycling; moreover, it has a relevant economic importance in aquaculture. The study aimed to assess the impact of polystyrene MPs and NPs on key organs: the midgut, testis sacs, and ovaries. Due to its hermaphroditic nature, E. albidus allowed us to estimate the impact of polystyrene MPs and NPs on both spermatogenesis and oogenesis within the same individual. Ultrastructural analysis revealed cellular abnormalities in all organs in MP- and NP-exposed groups, including lamellar body formation, autophagy, and mitochondrial damage. Fluorescence microscopy and flow cytometry indicated increased oxidative stress, DNA damage, and cell death. The midgut exhibited severe degeneration and disrupted homoeostasis, while reproductive organs, especially testes, showed significant DNA fragmentation and cell death processes. These findings underscore the negative effects of MP and NP polystyrene on minute soil invertebrates, which compromise intestinal and reproductive functions. They raise concerns about fertility and overall population viability in contaminated environments.