Species-specific bioaccessibility of microplastics by benthic macroinvertebrates

Since effect and bioaccumulation tests have so far mainly used monodisperse microplastics, while natural microplastics are diverse, better understanding of the actual biological availability of microplastics remains limited. Therefore, risk assessments based on such test data are questionable. In this study, we investigate species-specific uptake of an environmentally realistic microplastic mixture (ERMP) for 11 species. Species specificity is demonstrated by describing ingested particles as a continuum, quantified through continuous size distributions. Differences are statistically tested both among species data and against control distributions. The quantity and type of ingested particles are determined using a polymer- and size-specific blank correction procedure. The results indicate that adverse effects such as reduced growth and reproduction, previously observed in Lumbriculus variegatus after ERMP exposure, can be causally linked, as microplastics were detected in the species’ whole-body tissue samples. We discovered that only a select fraction of ERMP is bioavailable for Asellus aquaticus, Corbicula fluminalis, Corophium volutator, Hyalella azteca, Lumbriculus variegatus, Limecola balthica, Mytilus edulis and Tubifex spp. Most particles detected in organisms ranged from 5.5 to 27.7 µm, but differed in polymer composition, with 5.5 µm also being the method’s lower size-detection limit. The abundance of egested ERMP significantly exceeded that retained in tissue samples, indicating that organisms can egest most ingested microplastics within a 24 h depuration period. The lowest ERMP numbers were found in tissue and egestion samples of Chironomus riparius and Potamopyrgus antipodarum, respectively, averaging <1 particle/organism. The highest egested ERMP count occurred in M. edulis, at 49.9 particles/organism, primarily polystyrene. Similar size distributions were found for amphipods A. aquaticus and H. azteca, both sediment grazers/scavengers, and for clams C. fluminalis and L. balthica, both facultative deposit feeders. The particle size- and species-specific ingestion patterns highlighted here offer a critical foundation for understanding effect mechanisms such as particle volume-based food dilution.