Impacts of weathered microplastic ingestion on gastrointestinal microbial communities and health endpoints in fathead minnows (Pimephales promelas)

Microplastics are a ubiquitous presence 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 adsorb harmful toxins. Because lab-based studies on microplastics are often conducted with clean polymers, in ecologically unrealistic conditions and concentrations, the impact of these weathered microplastics on aquatic organisms in ecologically relevant conditions is still unclear. To help address the need for ecologically relevant microplastic exposure data, we incubated 500 micron polyethylene microplastic beads in Muskegon Lake, MI, USA and used them to conduct a 28 day ingestion study using fathead minnows (Pimephales promelas). We examined the effects on the fish gut microbial community along with hepatic gene expression and health parameters. We found that microplastic ingestion impacted growth in male fathead minnows. Microplastic treatment did not significantly alter the beta diversity of the gut microbial community for either sex, but there were clear differences between sexes and over time, indicating that these factors may outweigh the impacts of microplastic ingestion on beta diversity in the gut. The expression of immune response genes was not altered in males. It did, however, cause some changes to alpha diversity metrics in both sexes and there were several differentially abundant taxa among treatments. These data suggest that microplastic ingestion has health effects, but these effects may be sex specific across certain species and are likely not being solely driven by changes in gut microbial communities.