Bioavailability of micro/nanoplastics and their associated polycyclic aromatic hydrocarbons to Daphnia Magna: Role of ingestion and egestion of plastics

Aquatic ecosystems are ubiquitously polluted and deteriorated by micro/nanoplastics (MPs/NPs) and their associated contaminants. However, the bioavailability of MPs/NPs and their associated hydrophobic organic contaminants (HOCs) remains largely unknown. This study employs passive dosing systems to study the bioavailability of differently-sized MPs (3 and 20 μm)/NPs (80 nm) and their associated polycyclic aromatic hydrocarbons (PAHs) to Daphnia magna, a model species in aquatic ecosystem. At constant concentrations of freely dissolved PAHs, the presence of MPs/NPs raises the immobilization of D. magna to 71.1-80.0 %, far higher than their counterparts caused by PAHs (24.4 %) or MPs (20.0-24.4 %)/NPs (15.5 %). It demonstrates that the MPs/NPs-associated PAHs are bioavailable, acting as a key contributor (37.1-50.0 %) for the overall immobilization. Interestingly, although the immobilization of D. magna caused by MPs is higher than NPs, the bioavailability of MPs/NPs-associated PAHs declines with plastic size. Such a trend is due to the fact that MPs are actively ingested but hardly egested; while NPs are passively ingested and rapidly egested, leading to a continuous and higher accessibility of NPs-associated PAHs to D. magna. These findings clarify an integrated role of ingestion and egestion in controlling the bioavailability of MPs/NPs and their associated HOCs. Further, this study suggests that MPs/NPs-associated HOCs should be primarily concerned in chemical risk assessment in aquatic ecosystem. Accordingly, both ingestion and egestion of MPs/NPs by aquatic species should be addressed in future studies.