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.