The global distribution and anticipated increase of environmental microplastic (MP) pollution are concerning. However, while the impacts of macroplastic litter on wildlife are apparent, we know relatively little about the MP hazard potential. Moreover, the current ecotoxicological methodology is inadequate for solid waste particles and MP hazard assessment because it fails to distinguish particle and chemical effects. This thesis improves our understanding of the particle effects of MP relative to other microparticles.

First, a comparative analysis of effect studies on MP and mineral particulates across different biological organisation levels revealed high similarities in responses between these materials (Paper I). At the suborganismal levels, the similarity in the effect concentrations suggests shared particle effect mechanisms. At the higher levels, however, MP induced more severe impacts, possibly due to chemical leaching. Moreover, the highly variable MP effect concentrations motivated exploring the role of polymer properties and ageing status on MP effects; these aspects were addressed in Papers II-III.

In Paper II, the possibility of MP acting as a vector of contaminants was evaluated, showing enhanced transport of highly hydrophobic organic contaminants (HOC) at very high HOC and MP concentrations. However, observing it at environmentally relevant contaminant levels would be unlikely. 

Paper III compared behavioural and physiological responses in benthic amphipods to MP exposure using different polymers (polystyrene and polyethylene terephthalate) and clay as a non-plastic reference particle. The amphipods avoided sediments with high concentrations of the added material regardless of the material type, including aged and virgin MP and clay. 

Solid waste, including MP, co-occur with various suspended solids in aquatic environments; therefore, the natural solids can serve as reference material when evaluating the MP particle effect. In Paper IV, a novel method for testing MP effects in mixtures with reference particles was proposed. In the exposure experiment with daphnids, the method was used to derive hazard thresholds for the MP contribution to suspended matter conditional on the total suspended solid concentration in the water. 

Together, these studies add to our understanding of MP-biota interactions and suggest that similarly sized MP and natural particulates share similar particle effects. However, MP might have a higher potential as vectors of chemical contaminants, which needs to be further evaluated in environmentally relevant settings.

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 similar to 5 mu m) and kaolin clay (reference material; similar to 3 mu m). 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 Sigma PCBs between the microplastic-exposed and control animals. Also, higher size-specific egg production in the animals carrying PCB and receiving food mixed with micro-plastics 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.

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.

Microplastic pollution is currently perceived as an environmental hazard, and adverse effects have been reported at various levels of biological organization. However, most experimental designs do not allow plastic-specific effects to be distinguished from those caused by other particles, such as clay and cellulose, which are naturally ubiquitous in the environment. We suggest that microplastic effects reported in recent ecotoxicological studies are similar to those induced by the natural particles. To provide a sound basis for risk assessment, experimental designs must not only be able to disentangle the effects of food limitation and particle toxicity but also demonstrate whether microplastics cause impacts that differ from those induced by natural particles.

Trophic magnification factor (TMF) analysis employs stable isotope signatures to derive biomagnification potential for environmental contaminants. This approach relies on species delta N-15 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 delta N-15 and delta C-13 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 delta N-15 (+0.1 parts per thousand) and decrease in delta C-13 (-0.1 parts per thousand). 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.

Microplastics (MP) are an emerging type of solid contaminants ubiquitous in aquatic environments. Coastal, soft bottom ecosystems are particularly susceptible to MP pollution because they are deposition areas for solid and soluble contaminants. The addition of MP or any other inert particles to the sediment may dilute its nutritional content and modify structural integrity, leading to adverse effects in sediment-living biota. Behavioural endpoints are sensitive but underutilized indicators of habitat quality linked to physiological responses, such as metabolism, which is essential for understanding the mechanisms behind the MP effects. We studied the behaviour of the benthic deposit-feeding amphipod Corophium volutator exposed to sediments enriched in MP and measured the Electron Transport System activity (ETS) as a proxy for respiration in the animals facing varying sediment quality. The natural sediments were manipulated by dilution with two common plastic polymers (polystyrene PS and polyethylene terephthalate PET — aged and virgin) and kaolin clay as reference material at 1 and 10% sediment dry weight. We did not find any significant effects of neither material type nor ageing status on the behavioural traits. However, a significant increase in erratic non-feeding movements in the 10% mixture treatments suggested a general stress response to the altered habitat quality. There was also a consistent preference for the natural sediment, as evidenced by a relatively higher burrowing frequency compared to mixed sediments. The ETS activity was positively correlated with the increased physical activity due to the non-feeding movements but not food acquisition behaviour. In conclusion, the effects of MP on the behaviour and metabolic state of deposit-feeding amphipods are similar to other nutritionally inert particles, and the adverse effects are more of a reaction to a general disturbance than being material specific. Nevertheless, compared to the published data on MP effect concentrations in benthic organisms, behavioural endpoints are sensitive and informative when studying alterations in sediment quality; thus, they may provide complementary information for developing sediment quality criteria.