Species-specific bioaccessibility of microplastics by benthic macroinvertebrates

Effect and bioaccumulation tests have mainly used monodisperse microplastics, whereas natural microplastics are diverse. Consequently, understanding of their actual biological accessibility remains limited, which constrains risk assessment. The goal of this study was to investigate species-specific uptake of an environmentally representative microplastic mixture (ERMP; 9-5386 μm; PE, PP, PET and PS) in 11 freshwater and marine benthic macroinvertebrate species. Species-specific uptake was assessed by describing ingested particles as a continuum through continuous size distributions and comparing these distributions among species and against controls. Samples underwent enzymatic/hydrogen peroxide digestion and FTIR microscopy, followed by polymer- and size-specific blank correction. Adverse effects previously observed in the freshwater worm Lumbriculus variegatus could be linked more directly to exposure because microplastics were detected in post-depuration organism samples. Only a select fraction of the ERMP mixture was ingested by Asellus aquaticus, Corbicula fluminalis, Corophium volutator, Hyalella azteca, L. variegatus, Limecola balthica, Mytilus edulis and Tubifex spp. Most particles detected in organisms ranged from 5.5 to 27.7 μm and differed in polymer composition among species; 5.5 μm was also the method's lower size-detection limit. Egested ERMP significantly exceeded particles remaining in post-depuration organism samples, indicating substantial egestion during the 24 h depuration period. The lowest ERMP numbers occurred in post-depuration organism samples of the midge larva Chironomus riparius and egestion samples of the mud snail Potamopyrgus antipodarum, respectively, averaging <1 particle/organism. The highest egested count occurred in the mussel M. edulis, at 49.9 particles/organism, primarily polystyrene. Similar size distributions were found for the amphipods A. aquaticus and H. azteca, both sediment grazers/scavengers, and for the clams C. fluminalis and L. balthica, both facultative deposit feeders. These particle size- and species-specific ingestion patterns provide a basis for understanding effect mechanisms such as particle volume-based food dilution.