Effects of microplastics on survival, reproduction, and the microbiome of the freshwater oligochaete <i>Tubifex tubifex</i>

In freshwater ecosystems, particularly in riverine sediments, high levels of microplastics (MPs) (<5 mm) have been reported, potentially putting sediment-dwelling macroinvertebrates at risk. However, the effects of MPs on macroinvertebrates remain unclear, despite their critical roles in freshwater food webs. This study investigated whether exposure to three types of MPs (6 µm and 45 µm polystyrene microbeads, 100 µm polyethylene terephthalate (polyester) microfibers) causes adverse effects in the freshwater oligochaete, Tubifex tubifex. Worms were exposed across five environmentally relevant concentrations of each type of MP in sediment (negative control, 0.1, 1, 10, 100, and 1000 MPs per gram dry weight sediment). The exposures were also conducted using either pristine MPs or MPs that had first been colonized with biofilms. Survival of adult worms along with production of cocoons and juvenile worms were assessed following exposure to MPs for 28 days. Changes in the microbiota of whole oligochaetes and sediments were assessed using the V3-V4 regions of the 16S ribosomal RNA gene. Accumulation of MPs was examined after a gut clearance period to assess the ability of the worms to retain MPs. There were no statistically significant effects to survival or reproduction across all exposure concentrations and scenarios for the three types of MPs, along with no evidence of significant MPs accumulation. However, MPs exposure significantly altered host and sediment microbial communities, with effects varying by polymer type, particle size, and condition. In Tubifex tubifex, pristine 45 µm PS microbeads at the highest concentration increased Shannon diversity, while the highest concentration of biofouled 100 µm PET microfibers significantly altered beta diversity. In exposures using biofouled MPs, more differentially abundant bacterial genera were identified compared to pristine MPs, and included taxa linked to nutrient cycling and plastics degradation. In sediments, both alpha and beta diversity were significantly affected by 45 µm PS microbeads, and more differentially abundant bacteria were identified in these exposures compared to 100 µm PET microfibers. While chronic exposure to environmentally realistic MPs did not cause adverse effects to survival or reproduction in Tubifex tubifex, the consequences of alterations to the host microbiome due to MPs exposure requires further investigation.