Chemical Effects

The plastic materials are highly functional compounds of synthetic polymer and additives like plasticizers, flame retardants or colorants). The monomers, catalysts, stabilizers and additives leach to produce chemical toxicity [72].

Many of the chemicals associated with plastics have been identified as either toxic or endocrine disruptors, including bisphenol-A, phthalates such as di-n-butyl phthalate and di-(2-ethylhexyl) phthalate, polybrominated diphenyl ethers (PBDEs) and metals used as coloring agents [80,81,102,109]. These additives are weakly bound or not bound at all to the polymer molecule, and as such, these chemicals will leach out of the plastic over time. Such releases can be facilitated in environments where particle dispersal is limited and so the plastics will experience UV degradation and high temperatures [22]. The locations where microplastics may accumulate in soil and surface waters are then likely to be subjected to the possible release of these chemicals from plastics and are transferred to water, sediment and organisms. Locations closer to the source of plastic litter MPs are likely to have higher concentration of plastic additives. Lithner et al. [81] showed that different plastic items can leach toxic chemicals into water that can cause varying effects on Dapbnia magna [27].

For instance, with a liquid-to-solid (L/S) ratio of 10 and 24 h leaching time, leachates from polyvinyl chloride (PVC), polyurethane (PUR) and polycarbonate (PC) were found to be most toxic with EC50 values of 5-69 g plastic/L[81]. Higher L/S ratios and longer leaching times resulted in leachates from plasticized PVC. The epoxy resin products were the most toxic at EC50 of 2-235 g plastic/L. There are many challenges associated with the characterization of leachates owing to the potential diversity of physicochemical properties of chemical breakdown products [21].

Several laboratory studies depict the capacity of MPs to modify adverse effects of chemicals by affecting the bioavailability or acting as an additional stressor. Besseling et al. [82] observed a decreased bioaccumulation of polychlorinated biphenyls in lugworms at higher doses of PS particles. Karami et al. [78] modulate the impacts of phenanthrene on biomarker responses in African catfish (Clarias gariepinus). Paul-Pont et al. [83] detected the modulations of adverse effects by an exposure to phenanthrene-loaded low-density polyethylene (LDPE) fragments (African catfish) and PS beads and fluoranthene (Mytilus spp.), respectively. However, Gouin et al. [84] and Koelmans et al. [85] highlighted the minor influence of MPs as vectors for the bioaccumulation of pollutants considering they are outcompeted by naturally occurring matter.

In addition to the potential of MPs in influencing the bioavailability of toxic compounds, Besseling et al. [86] suggested that MPs can interfere with intra- and interspecies signaling (e.g., pheromones and kairomones) as an integral component of aquatic biocoenosis regulating predator-prey interactions as well as population and community structures [87]. It can lead to maladaptive responses in both the signaler and receiver [88]. But the fact still remains unclear whether MPs are the culprit for maladaptive responses since there exists an abundance of additional particulate organic and inorganic matter in aquatic ecosystems as well.

Biofilm-Related Impact

Colonization of MPs with microbes and adsorption of biopolymers increase the nutritional value and improve the “taste,” making them more attractive for biota in contrast to the colonization of MPs with pathogens [89] and toxic algae/bacteria, which might induce chemical toxicity or avoidance of “bad tasting” MPs.

Effect of Biofilm

  • • Biofilm formation increases the density of floating MPs and leads to sedimentation of these low-density particles [90].
  • • Formation of hetero-aggregates consist of particulate matter (MPs as well as other suspended solids) and microbes (e.g., protozoans, algae) with biopolymers acting as binders. Due to the abundance of microscopic particles, the availability to micro-feeders (e.g., protozoans, planktonic crustaceans) decreases, and large hetero-aggregates are accessible to macro-feeders (e.g., planktivorous fishes).
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