Interactions between plastic pollution, microorganisms and invertebrates in freshwater systems

Plastic pollution is ubiquitous throughout the environment and there is widespread concern over its impacts on aquatic ecological systems. Whilst work to understand the risks of plastic has traditionally focused on the physical and chemical characteristics of materials, the biological factors that may alter the impacts and fate of plastics have been largely overlooked, particularly within freshwater environments. Therefore, this thesis explored interactions between plastic litter, microbial communities and aquatic invertebrates within UK freshwater. Work to investigate the composition and metabolic functionality of microbial communities attached to plastic surfaces was conducted. The attraction of benthic invertebrates to microbially colonised plastic and their influence on the fate and potential impacts of plastic litter in the environment was also examined. Plastic surfaces hosted significantly distinct microbial communities compared to a non-plastic surface, and harboured communities with the potential to be pathogenic and to alter xenobiotic and biogeochemical cycling within the wider environment. These plastic-associated communities were also found to produce the odorous compound dimethyl sulfide. Given the role of this compound as a foraging cue in marine systems, this could significantly enhance the interactions which occur between plastic litter and aquatic organisms. Work to examine the influence of invertebrates on the fate of plastic in the environment found no significant interactions between microbially-colonised plastic and the amphipod Gammarus pulex. In contrast, the caddisfly larvae Agrypnia sp. rapidly fragmented plastic films (maximum diameter 6 mm) into hundreds of microplastics between 35.61 – 927.86 μm (maximum diameter), which is likely to have considerable implications for the bioavailability and fate of plastic litter in the environment. These findings demonstrate that to build an accurate and comprehensive understanding of the risks of plastic pollution in freshwater environments a better working knowledge of the biological factors which influence the fate and impacts of plastics is now urgently needed.