Microplastics are ubiquitous in the world’s aquatic environments, and their threat to aquatic biota is poorly understood, especially in freshwater ecosystems. In the environment, microbial biofilms can form on the surface of microplastics, which may sorb harmful toxins. While many laboratory-based studies use clean polymers under ecologically unrealistic conditions and concentrations, we incubated 500 μm polyethylene microplastic beads for 56 days in Muskegon Lake, Michigan, USA and used them to conduct a 28-day ingestion study with fathead minnows (Pimephales promelas). The study was conducted with a control group and two treatment groups, which received a low (4 beads) or high (16 beads) dose of weathered microplastics each day. We compared the treatment groups to the controls to assess the effects of weathered microplastic ingestion on growth (change in mass), condition factor, hematocrit, and fish gut microbial communities in both sexes. We also assessed the expression of three hepatic genes in males. Growth was lower in the high microplastic treatment group in male fathead minnows. The beta diversity of the gut microbial community was not impacted for either sex. There were some changes to alpha diversity metrics in males and several differentially abundant amplicon sequence variants (ASVs) in females. The expression of hepatic stress response genes was not altered in males. We also looked at the gut microbial community between sexes and over time within the control group and found clear differences, indicating that sex effects and environmental factors may have outweighed the impacts of microplastic ingestion on gut microbial diversity.
