Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Robert C Hale

Committee Member

Andrew R Wargo

Committee Member

Bongkeun Song

Committee Member

Wolfgang K Vogelbein

Committee Member

Patty Zwollo


There is mounting evidence that microplastics are a persistent and increasing hazard for aquatic organisms. The effects of microplastics on organisms and ecosystems are complex, however, and may be linked to a wide variety of particle characteristics including size, shape, polymer, additive chemistry, and degree of weathering. Assessing risk is complicated by the fact that many known effects of microplastics are sublethal, and that plastics have been postulated to interact with other stressors, such as pathogens. The work presented here expands our understanding of these complex effects. First, the impacts of microplastics on sedimentary microbial ecosystems and biogeochemical carbon and nitrogen cycles were investigated. A microcosm experiment using salt marsh sediment amended with polyethylene (PE), polyvinyl chloride (PVC), polyurethane foam (PUF) or polylactic acid (PLA) microplastics was conducted. We found that the presence of microplastics altered sediment microbial community composition and nitrogen cycling processes. Compared to control sediments without microplastics, PUF- and PLA-treated sediments promoted nitrification and denitrification, while PVC inhibited both processes. These results indicate that nitrogen cycling processes in sediments can be significantly affected by different microplastics, which may serve as organic carbon substrates for microbial communities. Second, we probed the virus-related mortality of a commercially important salmonid species under chronic exposure to nylon microfibers, polystyrene microplastics, and natural marsh grass microparticles. Mortality increased when fish were co-exposed to pathogen and microparticle, particularly nylon microfibers. This correlated with host viral load and mild gill inflammation. As such, we speculated that chronic exposure microplastics may create opportunities for pathogens to bypass defenses and colonize hosts via sensitive tissues. To investigate if this was enhanced by the physical properties of plastic microfibers, we assessed differences in mortality following chronic exposure to nylon microfibers and powder, finding that fibers had a greater effect than powdered counterparts. The importance of the timing of microplastic exposure was also confirmed by completing viral/microplastics co-exposures where microplastics were dosed before, after, or before and after viral introduction. Indeed, virulence was most enhanced when fish were exposed to microplastics pre-virus or chronically, significantly more so than post-virus only. Finally, we tested whether UV-weathering changed the effect of natural and plastic microparticles on disease-related mortality. We observed changes in the virulence effects of microparticles following UV-weathering, but the pattern of that change was inconsistent and merits further research. Considering their ubiquity and increasing concentrations globally, further research on the effects of microplastics is warranted. Particularly, the work here demonstrates that microplastics may influence entire communities and inorganic nutrient cycling systems, classifying microplastics as a potential planetary boundary threat. Further, we illustrate that even when microplastics alone may not have substantial effects on a fish population, when combined with disease they may amplify pathogen-related mortality significantly. More research on the interplay between microplastics and infectious disease is recommended, particularly as it may inform researchers on the risks of microplastics to human health.




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