Microplastics in river sediment: Chronic exposure of the amphipod Gammarus fossarum to polyethylene terephthalate in a microcosm

Microplastics constitute a form of particulate matter in aquatic environments, where they are a widespread pollutant. The broad range of particle sizes facilitates interactions with diverse species assemblages. Exposure to microplastics can negatively impact organisms, but similar effects also arise from exposure to naturally occurring particles, such as increased oxidative stress. It therefore remains uncertain, what effects are specific to microplastic particles, and how these effects manifest as a consequence of chronic exposure. Here we show in microcosm experiments that long-term exposure (111 days) to irregularly shaped polyethylene terephthalate (PET) fragments (10 to 400 µm) added to riverine sediments did not negatively impact the amphipod Gammarus fossarum's group size, and oxygen consumption, and minimally affected proteome composition. We found that these results were consistent for male and female specimens when exposed to an environmentally relevant concentration (0.004% of sediment dry weight; dw) and an environmentally less realistic one (4% dw). In female specimens' whole proteomes, we identified two highly differentially abundant proteins, which have been associated with an organism's response to xenobiotics. We conclude that in this sentinel species exposure to PET microplastic fragments mixed into the sediment does not elicit significant stress, even at concentrations exceeding current exposure levels in the environment. Microplastics are concerning pollutants, but their impact on aquatic life remains uncertain. Studies on PET, a common polymer, are limited. Most research involves high-dose, short-term exposure, making long-term risk assessment challenging. In our 111-day study, we exposed amphipods to PET microplastics in sand considering an environmentally realistic and an extreme concentration. Surprisingly, we observed minimal effects on measured variables, even at high doses. This suggests that chronic PET microplastic pollution in sediment may have limited immediate impacts on the assessed species.