Increasing the environmental relevance and realism of microplastic exposures changes the degree of toxicity to Daphnia magna

Microplastics (MP) are widely recognised as a contaminant of concern in the environment. Found in all environments sampled, the widespread use of plastic due to the versatile and relatively cheap cost of the material in combination with its long life by design, means that this is likely to be a problem facing the environment for the foreseeable future. Due to this, it is important to understand how microplastics might be impacting their environments, both in terms of physico-chemicals changes and also in terms of potential detrimental impacts to the organisms encountering the plastics. Laboratory based toxicity studies are widely used to explore the issues facing organisms by replicating these encounters. However, unlike chemical based toxicity tests, microplastics have a range of properties (such as hydrophobic surfaces) that may make this more challenging, and therefore previously established test protocols may need modifications to make them more applicable to this emerging pollutant. Establishing thorough reporting of baseline data and learning from similar fields, such as nanomaterial toxicity, will enable a greater understanding of the mechanistic toxicity and potential threat that MP poses to organisms and subsequently their wider ecosystems.
This thesis explores various elements of toxicity testing encompassing MP dispersal, medium influence on Daphnia response and finally mixture toxicity. By exploring the different methods of dispersing hydrophobic polyethylene beads, test designs can be modified to be more environmentally realistic whilst ensuring adequate dispersion (Chapter 3). The significant variation in both control (medium only) and chemical (sodium dodecyl sulphate) exposure response in Daphnia highlights the need to consider the testing medium during the experimental design stage for environmentally relevant test endpoints (Chapter 4). Finally, a combination of elements from Chapter 3 and 4 formed the study design for mixture exposure of three chemicals (sodium dodecyl sulphate, triclosan and diclofenac) in various media explored in Chapter 4, to Daphnia in chemical only and chemical with MP in combined exposures to ascertain how the effect of the chemical may vary (Chapter 5). Combining the various elements of this thesis explores the variability resulting from the study design within MP toxicity tests and highlights the need for environmental consideration at the design stage to expand the scope of MP laboratory-based toxicity studies to increase the environmental relevance and realism going forwards.