Date of Award
Doctor of Philosophy (PhD)
Dr. Peter van den Hurk
Dr. Cindy Lee
Dr. Charles Rice
Dr. Barbara Beckingham
Plastic materials have provided innovative solutions to society’s evolving needs and challenges. Due to their durability and resistance to degradation, plastics remain in the environment for long periods of time and can therefore be transported to many environmental compartments such as water, sediment, and biota. Microplastics (MPs) have been defined as synthetic plastic particles that have at least one dimension less than 5 mm and are insoluble in water. Microrubber (MR), a recently acknowledged sub-group of MPs, has been documented in environmental samples more recently, frequently comprising a large portion of total MPs of various samples. In South Carolina specifically, suspected tire wear particles (TWP) have been found abundantly in sediment and water in the Charleston Harbor area, potentially originating from road runoff near major highways and bridges. Recent studies on MR have demonstrated toxicity in a variety of organisms, but with mixed results due to the complexity of the composition of MR and differences in test conditions.
The goal of this research was to determine the toxicological impact of MR particles on aquatic organisms. To achieve this goal, the first objective addressed acute toxicity from exposure to MR particles and measured bile fluorescence as a biomarker for polycyclic aromatic hydrocarbon (PAH) absorption, metabolism, and biliary excretion, ethoxyresorufin-O-deethylase (EROD) activity as an indicator for cytochrome P450-1A (CYP1A) activity, and glutathione S-transferase (GST) activity as an indicator for oxidative stress. The second objective investigated toxicity under environmentally relevant conditions through a pulsed, chronic exposure. Immunohistochemistry (IHC) was utilized to visualize CYP1A induction in various tissues in addition to biomarker analyses for DNA damage, membrane damage, and oxidative stress. Finally, the third objective was to understand the environmental impact and fate of MR by analyzing MP abundance in biota collected from stormwater ponds.
The results from acute toxicity tests revealed that bile fluorescence increased and CYP1A activity is induced as MR concentration increased, suggesting that PAHs are leaching from MR particles in the aquatic environment. Two fish species were utilized in acute exposures, the estuarine fish Fundulus heteroclitus (mummichog) and freshwater fish, Pimephales promelas (fathead minnow). Partial mortality was observed in P. promelas suggesting potential greater toxicity in freshwater conditions compared to estuarine or marine environments.
Chronic toxicity tests were the first to our knowledge that utilized whole MR particles in exposures at environmentally relevant concentrations (< 0.2 g/L) in fish. Immunohistochemistry of F. heteroclitus gill, intestine, and liver indicated strong induction of CYP1A in gill and liver cells and vasculature, with mild induction in intestinal cells and suggests that aqueous exposure to MR and thus MR leachate exerted a more prominent response compared to ingestion of particles themselves. Additionally, bile fluorescence increased as MR concentration increased as observed in acute exposures. Other biomarker tests indicated that antioxidant defenses were upregulated to prevent cellular damage as measured through an increase in the DNA damage byproduct 8-hydroxy-2’-deoxyguanosine (8-OHdG), a decrease in malondialdehyde (MDA) production indicating less lipid peroxidation, and an increase in the antioxidant, glutathione (GSH) when MR concentration increased.
Finally, the abundance of MP and MR in field-collected organisms from coastal stormwater ponds was measured by visual microscopy of digested animals. Stormwater ponds are hot spots for environmental pollution, including MR from road runoff. The majority (>80%) of MP recovered from biota across all sampling sites were suspected tire particles. The average number of MP per individual ranged from 0.3 to 71 MP and the average number of suspected tire particles per individual ranged from 0 to 57.7 tire particles. There were significant differences observed in the number of MP per individual between sites and between species. A combination of factors such as availability of MPs based on surrounding land use, stormwater pond dynamics, organism size, and organism feeding habitat influenced the total MP observed.
Overall, these data indicate that MR particles and their associated compounds exert a toxic effect on aquatic species and that stormwater ponds serve as a sink for MR accumulation in the environment.
LaPlaca, Stephanie, "Toxicity and Effects of Tire Crumb Rubber in the Aquatic Environment" (2021). All Dissertations. 2959.
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