Community dynamics and functional traits drive microplastic sequestration by marine nematodes

Abstract Benthic sediments are recognized as the principal sink for the 'missing plastic', yet the biological processes governing microplastic (MP) fate within this reservoir remain critically unresolved. Here, we investigate whether and how marine nematode communities, the most abundant metazoans in marine sediments, influence the fate and ecological risk of MPs. Using short-term (9-day) and long-term (30-day) microcosm experiments, we assessed the influence of MP density and exposure time on nematode community structure, functional diversity, and sequestration capacity. Our results reveal that MP sequestration by nematodes only manifests at very high-MP densities. Counter-intuitively, the most severe impacts on nematode communities occurred at low MP densities, despite particle uptake remaining below detection there. The magnitude of the sequestration role of the nematode community heavily depended on nematode functional characteristics, with opportunistic non-selective ingesters being responsible for 90% of the MP sequestration in the short-term experiment. With a structurally very different community at the start, MP uptake in the long-term experiment was considerably lower and shared by a functionally more diverse consortium of more K-selected taxa with diverse feeding strategies, but all having fairly wide buccal cavities. Our study reveals that nematode communities are an active and dynamic biological compartment where MPs are processed, transformed, and reintroduced into the food web, but the quantitative importance of their role in trophic transfer appears very limited.