The bioaccumulation effects of microplastics and associated organic pollutants in the aquatic environment

Microplastics are ubiquitous in the aquatic environment. Due to their high hydrophobicity, microplastics are usually loaded with different kinds of organic pollutants, whose bioaccumulation and toxicity to aquatic organisms will also be affected with the presence of microplastics. Consequently, bioaccumulation and biomagnification of microplastics and their associated organic pollutants, are often inferred to occur in aquatic food webs. The effects of microplastics on the bioaccumulation of organic pollutants do not have a definite conclusion yet, because results from laboratory studies supporting bioaccumulation are hampered by using unrealistic exposure conditions and results from modelling studies are lack of verification. The terms of bioaccumulation and biomagnification are usually used for traditional pollutants, but there is no clear definition for microplastics yet, which will be discussed in detail in this review. Presented here are the results of a systematic literature review to examine whether current, published findings support the premise that microplastics and associated organic pollutants bioaccumulate in aquatic organisms or biomagnify within food webs. First, field and laboratory-derived data on aquatic species were collected from publications, and the bioaccumulation factors (BAF) were estimated across all species including different feeding types and habitat conditions. Second, we summarized the widely investigated factors for the accumulation phenomenon of microplastics in the relevant exposure systems, from publications until 2020. Current literature suggests that the bioaccumulation amounts in organisms depend on microplastic concentration, particle size, exposure time, particle shape and polymer type. Moreover, animal body size and feeding type can also affect the ingestion amounts of microplastics in organisms. Third, three possible scenarios of associated organic pollutants bioaccumulation along with microplastic ingestion were classified and potential mechanisms were discussed. The presence of microplastics can increase, decrease, or not alter the bioaccumulation of co-existing organic pollutants, depending on the fugacity diffusion ladder and environmental matrix effects. Finally, environmental influential factors, such as pH, salinity, and environmental aging, that can affect the bioaccumulation of organic pollutants were listed. In conclusion, we summarized the bioaccumulation characteristics of microplastics in aquatic organisms under both field and laboratory exposure conditions; analyzed the key factors that affecting the bioavailability of microplastics; explored the influences of microplastics in the bioavailability of co-existing organic pollutants; evaluated the important aquatic environmental factors that can affect the bioaccumulation of microplastics and co-existing organic pollutants. This paper will assist us to understand the accumulation phenomenon of microplastics in the aquatic ecosystems, microplastics effects on the transport of co-existing pollutants. Moreover, it will also provide basis for the risk assessment and policy formulation of microplastics. This review highlights the need for combined field-based and experimental studies to elucidate the possible routes of uptake of microplastics (and associated organic pollutants) and provides scientific guidance for future microplastics bioaccumulation studies.