| Microplastics are ubiquitous in the aquatic environments.Due to their high hydrophobicity,microplastics are usually loaded with organic pollutants,whose bioaccumulation and toxicity to different trophic levels of aquatic organisms are also affected by the presence of microplastics.Organisms constantly ingest microplastics directly from the environment or indirectly via trophic transfer due to the pervasiveness of microplastic pollutants.Consequently,bioaccumulation and biomagnification of microplastics and their associated pollutants,are often inferred to occur in aquatic food webs.The effects of organisms with different trophic levels on the bioaccumulation of microplastics are not well known,because the results from laboratory studies on bioaccumulation are hampered using unrealistic exposure conditions and the results from field sampling studies are lack verification.The bioaccumulation and biomagnification are usually used for traditional pollutants,but there are still no clear definitions for microplastic.Combining field investigations and laboratory exposure studies,we summarized the bioaccumulation and toxicity effects of microplastic and associated organic pollutants in the organisms with different trophic levels.Combining field investigations and laboratory exposure studies,several conclusions have been obtained as follows:（1）In order to trace the distribution of microplastics in surface water and organism species with different trophic levels from the Changjiang River Estuary,we investigated microplastic pollution in surface water,two wild gastropod species and seven wild fish species captured from three stations in the Changjiang Estuary and its adjacent area.The average abundance of microplastics in surface water was 660.7±220.5 items/m~3.Meanwhile,the average abundance of microplastics found in gills（1.1±0.4 items/individual）and gastrointestinal tracts（GIT,0.3±0.1items/individual）,respectively.The bioaccumulation factors were estimated from all species including different species and feeding types in our present study.Our results suggested that microplastics are possible to be aggregated in the higher trophic level（fish species）throughout the marine food web.Furthermore,the results suggested that predatory fishes might be suitable microplastics indicate microplastics in the environment.Using stable nitrogen isotope(δ15N)analysis,we identified the trophic levels of organisms,and illuminated the bioaccumulation and trophic transfer of microplastics in the Changjiang Estuary ecosystems.The results showed that there was a significant correlation between the concentration of microplastic and the trophic levels of the organisms.Therefore,there was potential biomagnification of microplastic in gastropod and fish species in the Changjiang Estuary ecosystems.（2）Considering that few studies focused on the contribution of trophic transfer,our study chose daphnids（achieved bioconcentration equilibrium after 24 h）exposed to 1 mg/L microplastics as a food source of D.rerio for the subsequent foodborne exposure experiment.The range of polyethylene microplastics concentration was 40–47μm,which was closed to the environmentally relevant microplastic concentrations.The results showed that the accumulation of microplastics resulting from foodborne exposure（0.01±0.01μg/mg）was significantly lower than that through waterborne exposure（0.06±0.02μg/mg）,suggesting microplastic bioaccumulation occurred mainly through direct environmental uptake rather than trophic transfer.However,more sublethal impacts,including abnormal hyperactive swimming behavior（107±5%induction）and oxidative stress（139±27%induction）,were observed in the foodborne group compared with waterborne group.Additionally,ingenuity pathway analysis（IPA）predicted both exposure routes caused obvious nervous system interference but through opposite modes of action.This was further verified by the alteration of neurotransmitter biomarkers,that dopamine and 5-hydroxytryptamine levels and cholinergic system changes were responsible for neurotoxicity following foodborne and waterborne exposure,respectively.Although the neurotoxic effects of microplastics may be species-and plastic-specific,the non-negligible effects deserve more further attention.（3）Current studies have shown that there are few studies on the migration of pollutants such as flame retardants carried by microplastics on the food chain.Meanwhile,there is no distinct conclusion about the toxic effects of organic pollutants carried by microplastics on organisms at different trophic levels.Therefore,we selected 20μm polyester fiber and tris-（2,3-dibromopropyl）isocyanurate（TBC）to exposure D.magna.The 24-hour exposure to microfibers and TBC alone and combined exposure to D.magna（achieved bioconcentration equilibrium after 24 h）in the real environment.Then,D.magna was used as a food source for the foodborne exposure of zebrafish,in order to explore the bioaccumulation and toxic effects of microfibers and co-existing pollutants（TBCs）in the foodborne on zebrafish.The results showed that the presence of microfibers did not significantly enhance TBC bioaccumulation,even after depuration.This is probably because TBC are easily released by fish,and microfibers could also insignificantly reduce the accumulation of TBC in individuals at lower trophic level（D.magna）.In the previous chapter,the accumulation of microplastics was found to cause neurotoxicity in zebrafish.Here,the results also showed the effects of microfibers and TBC exposure alone and in combination on the nervous system of zebrafish.This study found that zebrafish showed higher locomotion ability in both group of microfiber exposure alone（113.8±2.1%）and the group of combined exposure（152.8±14.1%）than TBC exposure alone（78.2±1.9%）.In addition,microfibers probably affected the activity levels of various enzymes in the cholinergic nervous system,while the contents of neurotransmitters in the dopamine nervous system would significantly increase when TBC was exposed alone.Therefore,microplastic fibers exposure can cause the neurotoxicity in zebrafish acting on enzyme activity and neurotransmitter secretion levels in the cholinergic nervous system,and neurotoxicity might be dispensable for triggering enhanced locomotory activity for zebrafish.Combining the field and laboratory-derived data（including microplastic and associated pollutants）on aquatic species,this research found that microplastics were transferred through different trophic levels.Although there is no obvious bioconcentration and biomagnification appearing in high trophic-level organisms,the few microplastics accumulated in organisms’bodies still damaged their physiological functions.Our results provide relevant toxicological data for our in-depth understanding of the accumulation of microplastics in the aquatic ecosystem and the possible ecological risks when they are transmitted with the trophic levels. |
