In aquatic ecosystems, microplastics are a relatively new anthropogenic substrate that can readily be colonized by biofilm-forming organisms. To examine the effects of substrate type on microbial community assembly, we exposed ambient Baltic bacterioplankton to plastic substrates commonly found in marine environments (polyethylene, polypropylene and polystyrene) as well as native (cellulose) and inert (glass beads) particles for 2 weeks under controlled conditions. The source microbial communities and those of the biofilms were analyzed by Illumina sequencing of the 16S rRNA gene libraries. All biofilm communities displayed lower diversity and evenness compared with the source community, suggesting substrate-driven selection. Moreover, the plastics-associated communities were distinctly different from those on the non-plastic substrates. Whereas plastics hosted greater than twofold higher abundance of Burkholderiales, the non-plastic substrates had a significantly higher proportion of Actinobacteria and Cytophagia. Variation in the community structure, but not the cell abundance, across the treatments was strongly linked to the substrate hydrophobicity. Thus, microplastics host distinct bacterial communities, at least during early successional stages.

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The occurrence of microplastic in the environment is of global concern. However, the microplastic hazard assessment is hampered by a lack of adequate ecotoxicological methods because of conceptual and practical problems with particle exposure. In the environment, suspended solids (e.g., clay and cellulose) in the same size range as microplastic, are ubiquitous. Therefore, it must be established whether the addition of microplastic to these background levels of particulate material represents a hazard. We present a novel approach employing a serial dilution of microplastic and reference particles, in mixtures, which allows disentangling the effect of the microplastic from that of the other particulates. We demonstrate the applicability of the method using an immobilization test with Daphnia magna exposed to polyethylene terephthalate (test microplastic; median particle diameter ~5 µm) and kaolin clay (reference material; ~3 µm). In the range of the suspended solids test concentrations (0–10 000 mg L −1 ), with microplastic contributing 0–100% of total mass, the LC 50 values for the plastic mixtures were significantly lower compared to the kaolin exposure. Hence, the exposure to polyethylene terephthalate was more harmful to the daphnids than to the reference material alone. The estimated threshold for the relative contribution of the test microplastic to suspended matter above which significantly higher mortality was observed was 2.4% at 32 mg of the solids L −1 . This approach has a potential for standardization of ecotoxicological testing of particulates, including microplastic.

The role of microplastic (MP) as a carrier of persistent organic pollutants (POPs) to aquatic organisms has been a topic of debate. However, the reverse POP transport can occur if relative contaminant concentrations are higher in the organism than in the microplastic. We evaluated the effect of microplastic on the PCB removal in planktonic animals by exposing the cladoceran Daphnia magna with a high body burden of polychlorinated biphenyls (PCB 18, 40, 128 and 209) to a mixture of microplastic and algae; daphnids exposed to only algae served as the control. As the endpoints, we used PCB body burden, growth, fecundity and elemental composition (%C and %N) of the daphnids. In the daphnids fed with microplastic, PCB 209 was removed more efficiently, while there was no difference for any other congeners and ΣPCBs between the microplastic-exposed and control animals. Also, higher size-specific egg production in the animals carrying PCB and receiving food mixed with microplastics was observed. However, the effects of the microplastic exposure on fecundity were of low biological significance, because the PCB body burden and the microplastic exposure concentrations were greatly exceeding environmentally relevant concentrations.

Trophic magnification factor (TMF) analysis employs stable isotope signatures to derive biomagnification potential for environmental contaminants. This approach relies on species δ(15)N values aligning with their trophic position (TP). This, however, may not always be true, because toxic exposure can alter growth and isotope allocation patterns. Here, effects of PCB exposure (mixture of PCB18, PCB40, PCB128, and PCB209) on δ(15)N and δ(13)C as well as processes driving these effects were explored using the cladoceran Daphnia magna. A two-part experiment assessed effects of toxic exposure during and after exposure; juvenile daphnids were exposed during 3 days (accumulation phase) and then allowed to depurate for 4 days (depuration phase). No effects on survival, growth, carbon and nitrogen content, and stable isotope composition were observed after the accumulation phase, whereas significant changes were detected in adults after the depuration phase. In particular, a significantly lower nitrogen content and a growth inhibition were observed, with a concomitant increase in δ(15)N (+0.1 ‰) and decrease in δ(13)C (-0.1 ‰). Although of low magnitude, these changes followed the predicted direction indicating that sublethal effects of contaminant exposure can lead to overestimation of TP and hence underestimated TMF.