Exploring the ecotoxicity of microparticle debris

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

A novel method for assessing microplastic effect in suspension through mixing test and reference materials

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 LC50 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

Microplastic-mediated transport of PCBs? A depuration study with Daphnia magna.

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

Growth Retardation and Altered Isotope Composition As Delayed Effects of PCB Exposure in <i>Daphnia magna</i>

Trophic magnification factor (TMF) analysis employs stable isotope signatures to derive biomagnification potential for environmental contaminants. This approach relies on species δ¹⁵N 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 δ¹⁵N and δ¹³C as well as processes driving these effects were explored using the cladoceran <i>Daphnia magna.</i> 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 δ¹⁵N (+0.1 ‰) and decrease in δ¹³C (−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