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Bioavailability of micro/nanoplastics and their associated polycyclic aromatic hydrocarbons to Daphnia Magna: Role of ingestion and egestion of plastics
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Using a passive dosing system that kept dissolved pollutant concentrations constant, researchers showed that microplastics and nanoplastics dramatically increase the toxicity of polycyclic aromatic hydrocarbons (PAHs) to the water flea Daphnia magna: immobilization reached 71-80% when MPs/NPs and PAHs were combined, compared to 24% for PAHs alone. The PAHs adsorbed onto microplastic surfaces were bioavailable and contributed 37-50% of the total toxic effect, acting as a vector that delivers concentrated doses of carcinogenic compounds to organisms that ingest the particles. These findings reveal that the true hazard of microplastics in polluted water is substantially greater than either the particles or the chemical contaminants would cause on their own.
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.
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