Microplastics as vectors of chemical contaminants and biological agents in freshwater ecosystems: Current knowledge status and future perspectives

Microplastics (MPs) are becoming ubiquitous, and their environmental fate is becoming an issue of concern. Our review aims to synthesize current knowledge status and provide future perspectives regarding the vector effect of MPs for chemical contaminants and biological agents. The evidence in the literature indicates that MPs are a vector for persistent organic pollutants (POPs), metals and pharmaceuticals. Concentrations of chemical contaminant in orders of six-fold higher on MPs surfaces than in the surrounding environmental waters have been reported. Chemical pollutants such as perfluoroalkyl substances (PAFSs), hexachlorocyclohexane (HCHs) and polycyclic aromatic hydrocarbons (PAHs), exhibiting polarities in the range of 3.3-9 are the commonest chemicals reported on MP surfaces. Regarding metals on MPs including chromium (Cr), lead (Pb), cobalt (Co), the presence of C-O and N-H in MPs promote a relatively high adsorption of these metals onto MP surfaces. Regarding pharmaceuticals, not much has been done, but a few studies indicate that commonly used drugs such as ibuprofen, ibuprofen, diclofenac, and naproxen have been associated with MPs. There is sufficient evidence supporting the claim that MPs can act as vectors for viruses, bacterial and antibiotic-resistant bacteria and genes, and MPs act to accelerate horizontal and vertical gene transfer. An area that deserves urgent attention is whether MPs can act as vectors for invertebrates and vertebrates, mainly non-native, invasive freshwater species. Despite the ecological significance of invasive biology, little research has been done in this regard. Overall, our review summarises the state of the current knowledge, identifies critical research gaps and provides perspectives for future research.