Dynamic fate of polystyrene microplastics in terrestrial snails through dietary exposure: accumulation, excretion, and molecular impacts

Microplastic (MPs) pollution poses significant environmental and health risks due to its widespread presence and persistence in ecosystems. Despite growing concerns, there is a lack of comprehensive studies investigating the mechanistic cascade governing the dynamic accumulation and toxicological fate of MPs in terrestrial organisms, especially in mollusks. Here, the dietary transfer of polystyrene (PS) MPs to edible snails (Achatina fulica) was investigated to explore effects of particle size and exposure concentration of MPs on the bioaccumulation, tissue distribution, clearance and the subsequent disruption of the host-microbiota axis. Our findings indicate that internal MPs load is primarily driven by exposure concentration, although particle size influences retention efficiency. Smaller MPs (2 μm) reached higher accumulation levels than 20 μm MPs, particularly at high concentrations where the gap reached 91.34%, with all detected particles localized exclusively within the digestive gland. Exposure to 2 μm PS MPs triggered a size dependent toxicological cascade, characterized by a 177.48% surge in malondialdehyde (MDA) levels and a significant collapse of the enzymatic antioxidant barrier, specifically the activities of superoxide dismutase (SOD) and catalase (CAT). Additionally, the molecular indicators of intestinal damage were more pronounced in these snails than in those exposed to 20 μm MPs. MPs exposure increased the complexity and connectivity of microbial co-occurrence networks, primarily driven by the expansion of Proteobacteria. Transcriptomics further showed that MPs exposure disrupted cell membrane integrity and activated immune defense and apoptotic pathways, ultimately leading to a critical imbalance in host immune-metabolic homeostasis. Our findings highlight that MP toxicity is a dynamic process driven by the size-retention-transformation axis, providing new insights into the fate and integrated biological risks of dietary MPs in terrestrial herbivores.