Microplastic particle characteristics influence effects on microbe-chironomid-arvae interactions

Through shared particle characteristics, microplastic (MP) particles are likely to contaminate freshwater fine particulate organic matter (FPOM) pools, which are the primary food source for particle-feeding organisms. However, key differences between MP particle types, including particle shape and polymer, may influence responses from the microbial community and particle consumers. Furthermore, relationships between FPOM-dependent particle-feeding organisms and their associated processes may be altered by MP exposure. We investigate the influence of MP (25-63 um) particle concentration (0, 1000, 50000 particles/kg sediment), shape (sphere, fragment and fibre) and polymer (polyethylene, polyethylene terephthalate, polypropylene, polystyrene) on microbial communities, a model particle feeder (larvae of the non-biting midge Chironomus riparius) and associated ecosystem processes. Our results show that MP exposure led to responses from the various levels of the ecosystem, but that these effects often depended on particle shape, polymer and concentration. MP fragments were associated with increase chironomid lipid content and microbial respiration (PET MPs at low concentration). Polypropylene MPs reduced early microbial abundance and chlorophyll- a concentration (at high concentration in the absence of chironomids), and increased microbial respiration (at high concentration). MPs per se, at a high concentration, also led to increased final microbial abundance (in the presence of chironomids) and chironomid biomass. Effects were especially associated with fragment and polypropylene MPs, characteristics that promote biofilm growth and hetero-aggregate formation. It highlights the importance of microbial responses towards MP exposure, as effects frequently reverberate through the ecosystem.