The Effects of Environmental Factors on the Toxicity of Nanomaterials in Fish

Nanomaterials (NMs) are materials that are between 1 and 100 nm at least one dimension. While natural processes (volcanos, lightning, erosion, etc.) have been producing NMs for the entirety of the earth’s existence, recent advances in material sciences have produced novel engineered NMs developed and manufactured for specific purposes in consumer goods and industrial applications. Due to the widespread use and rapidly increasing production of NMs, more engineered NMs will inevitably be released into the environment through indirect or direct release and thus will interact with environmental factors and organisms. This has raised concern with regulators and the public alike regarding the potential hazard of these engineered NMs. Another recent source of NMs results from the production, release, and subsequent environmental breakdown of plastics, leading to the presence of nanoplastics (NPLs) now ubiquitously found in aquatic environments. My thesis examines the potential for adverse effects of both engineered NMs and NPLs, with a specific focus on the interactive effects of these materials in the presence of modulating environmental factors such as ultraviolet (UV) light and natural organic matter on the toxicity/hazard o fishes. First, the effects of cerium oxides nanoparticles (CeO2 NPs) were examined using the Amazonian fish, the cardinal tetra (Paracheirodon axelrodi) as a model organism. Toxicity was examined either under laboratory light or UV light conditions with or without the addition of natural organic matter (NOM) from Rio Negro Amazon River water. Sub-lethal effects such as lipid peroxidation and gill malformations were evaluated and potentiation of toxicity by UV light and mitigation of these effects by NOM was demonstrated. In the second series of investigations, zebrafish (Danio rerio) embryos were exposed to novel nano-enabled pesticide formulations azoxystrobin (nAz) or conventional azoxystrobin (Az) either under laboratory light or UV light. Evaluation of multiple endpoints including lethality, oxygen consumption, yolk consumption, changes in antioxidant enzyme activity, alternations in gene expression and lipid peroxidation demonstrated significant potentiation of toxicity by UV light and a mechanism for the generation of reactive oxygen species under UV light as the potentiating factor was proposed. Similar to engineered nanomaterials, the potential hazard of NPLs as carriers of organic materials (the so-called “Trojan Horse” hypothesis) was then investigated using both zebrafish embryos and rainbow trout fingerlings (Oncorhynchus mykiss) as model species. Specifically, embryos or fingerlings exposed to NPLs either alone or coated with a model polycyclic aromatic hydrocarbon (PAH) phenanthrene were used to demonstrate the carrier function of NPLs for organic pollutants. Also investigated were the modulating effects of NOM on the noted potentiation of PAH uptake in rainbow trout fingerlings by NPLs. The results presented in this thesis represent 1) the first study to demonstrate the UV-induced sub-lethal effects of CeO2 NPs on fish at low environmental pH (~4.5), including the protective properties of NOM to reduce the noted UV-induced toxicity, 2) the first comprehensive and systematic report on the UV-induced lethal and sub-lethal effects of pesticide on zebrafish embryos, including demonstration of much higher toxicity associated with exposure to nano-enabled formulations when compared to conventionally applied pesticides, 3) the first comprehensive research to show both sorptions of phenanthrene onto the surface of NPLs and the carrier function of NPLs for facilitating the uptake of phenanthrene by zebrafish embryos and rainbow trout fingerlings. As part of this study, I also demonstrated mitigating effects of NOM on the co-contaminant carrier function of NPLs. My thesis improves our understanding of the effects of environmental factors on the toxicity of various NMs and NPLs, including the potentiation of hydrophobic organic pollutant uptake by NPs by the “Trojan Horse” effect. This thesis will provide much-needed data for regulators, risk assessors and the public to evaluate the potential impact of NMs and NPLs in the environment.