Contemporary concentrations of microplastics in aquatic ecosystems correlate with molecular stress responses in fish

Microplastics are a pervasive pollutant with impacts on organisms across ecosystems. As species increasingly live in the presence of plastic pollution, there is a need to deepen our understanding of biological responses to microplastics. To this end, we performed an experiment exposing fathead minnows (Pimephales promelas) to different microplastic treatments. We tested two polyethylene microplastic concentrations, reflecting current and predicted future conditions, of two plastic origins: pre-consumer plastic (never exposed to natural environments) and plastic gathered from Lake Ontario. We performed weighted gene co-expression analysis on liver tissue using directional mRNA sequencing data to evaluate gene expression among different treatments across sexes and to explore correlations with ecotoxicological metrics. We addressed the following questions: 1) is there a significant effect of microplastic exposure on gene expression, 2) if there are changes in gene expression attributed to microplastic exposure, do these effects differ across plastic concentration and/or plastic origin, and 3) does the effect of microplastics exposure vary by sex? Our findings provide evidence of metabolic changes in fish exposed to microplastics. Many of the gene modules found in our analysis correspond to cellular stress responses, further increasing the evidence that microplastic exposure has molecular effects on organisms. Furthermore, we found these molecular effects differed by sex, with a greater response found in females. As fathead minnows are an important model species, these results have important implications across aquatic species and ecosystems. Understanding how persistent pollutants, such as microplastics, may stress organisms across levels of biological organization will be key to mitigating anthropogenic change at various levels, from the molecular to the population and community levels.