Microplastics accumulation and microbiome alterations in the freshwater pulmonate snail, Planorbella pilsbryi

Microplastics (MPs) are widespread in freshwater systems, particularly in riverine sediments, where they may threaten sediment-dwelling organisms such as freshwater gastropods. Despite their ecological significance, the effects of MPs on these organisms and their potential accumulation, remain largely unclear. This study investigated the effects of three types of MPs (6 µm and 45 µm polystyrene [PS] microbeads, and 100 µm polyethylene terephthalate [PET] microfibers) using the freshwater pulmonate snail, Planorbella pilsbryi, under five concentrations spanning environmentally relevant to elevated exposure levels (0, 0.1, 1, 10, 100, and 1,000 MPs/mL water). Both pristine and biofouled (microbially colonized) MPs were used to determine accumulation within the organisms and effects on mortality, reproduction, and host-associated microbiomes. Microplastics accumulation was assessed post-exposure, and a gut clearance period was incorporated to assess excretion efficiency. Reproduction was assessed by counting egg clusters, and whole-body bacterial communities were analyzed using the V3-V4 regions of the 16S ribosomal RNA gene. Snails ingested all PS microbead types, with amounts increasing with exposure concentration, with more biofouled than pristine microbeads. However, most ingested MPs were efficiently excreted, with retention proportional to concentration. While survival and reproduction were unaffected, MPs exposure altered microbial communities. High concentrations (1,000 MPs/mL) of pristine 45 µm PS microbeads significantly increased alpha diversity, while beta diversity was significantly different from controls at high and moderate (10 MPs/mL) concentrations in biofouled 45 µm PS microbead exposures, and at high concentrations in pristine 45 µm PS microbead exposures. Differential abundance analysis showed that certain bacterial genera responded only to microbeads or microfibers, while others were affected across both particle types. Overall, our findings demonstrate that substantial ingestion and retention of MPs can occur in Planorbella pilsbryi without significant effects on survival or reproduction, while still eliciting changes in the host-associated microbiome that vary with particle characteristics.