Impacts to Larval Fathead Minnows Vary between Preconsumer and Environmental Microplastics.

Microplastics are a complex suite of contaminants varying in size, shape, polymer, and associated chemicals and are sometimes referred to as a "multiple stressor." Still, the majority of studies testing hypotheses about their effects use commercially bought microplastics of a uniform size, shape, and type. We investigated the effects of polyethylene and polypropylene microplastics purchased as preproduction pellets (referred to as "preconsumer") and a mixture of polyethylene and polypropylene collected from the environment (environmental microplastic). Embryo-stage fathead minnows were exposed to either the physical plastic particles and their leachates or the chemical leachates alone at an environmentally relevant (280 particles/L) or high (2800 particles/L) concentration for 14 d. The effects of microplastics differed by polymer type and presence of environmental contaminants, and effects can be driven by the physical particles and/or the chemical leachates alone. Larvae exposed to preconsumer polyethylene experienced a decrease in survival, length, and weight, whereas preconsumer polypropylene caused an increase in weight. Environmental microplastics caused a more drastic increase in length and weight and almost 6 times more deformities as the preconsumer microplastics. Although preconsumer microplastics caused effects only when organisms were exposed to both the particles and the chemical leachates, the environmental microplastics caused effects when organisms were exposed to the chemical leachates alone, suggesting that the mechanism of effects are context-dependent. The present study provides further support for treating microplastics as a multiple stressor and suggests that testing for effects with pristine microplastics may underestimate the true effects of microplastics in the environment. Environ Toxicol Chem 2022;41:858-868. © 2021 SETAC.