Date of Award

12-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Advisor

Arai, Yuji

Committee Member

Skipper , Horace

Committee Member

Kerrigan , Julia

Abstract

The worldwide production of engineered nanoparticles (ENPs) has grown rapidly in our era because of the popular applications in industrial and consumer markets. Especially silver (Ag) NP is one of mostly common ENPs in our consumer products due to its unique antimicrobial property. However, release of AgNPs into environment through wastewater treatment plants poses a question that what impact AgNPs have on microbially mediated processes in environment (e.g., nutrient and trace element cycles). In the last decades, numerous toxicological studies of AgNPs on bacteria have been conducted in pure culture systems, and several different antimicrobial mechanisms of AgNPs have been proposed. The toxicity of AgNPs is generally caused destabilization of the cellular outer membrane or by the release of Ag+ via dissolution of NPs. Ionic silver is a well-known anitimicrobial agent. It can readily react with amino acids in the cellular structure. Other mechanisms are the inhibition of several oxidative enzymes (e.g., hydroxylamine oxidoreductase-specific oxygen uptake rates and ammonia monooxygenase-specific oxygen uptake rates), surface binding and damage to membrane, suppressing DNA replication abilities, and generation of reactive oxygen species. While these mechanisms are accepted knowledge in the field, it is not clearly understood whether AgNPs exhibit the similar degree of toxicity to bacteria in the heterogeneous soil and water environment. In this study, the impact of AgNPs (50nm uncoated and 15nm PVP-coated AgNPs) including ionic silver (Ag+) to soil nitrifying bacteria was investigated along with batch sorption and dissolution experiments. Nitrifying bacteria were chosen as a model microbial community that serves a critical role in sustaining the global N cycles in the terrestrial environment. The results of nitrification potential (i.e., kinetic rate) suggest that Ag+/AgNPs, which strongly sorb in soils, suppressed the nitrification processes. Interestingly, the antimicrobial effect was dependent on chemical forms and concentrations, but mostly dependent on concentration. The observed toxicity to nitrification process in this study was in the order: PVP-coated 15nm AgNPs > ionic silver > uncoated 50nm AgNPs. Unlike the toxicity data reported in the pure culture systems, the results of this study suggest that toxicity of AgNPs to nitrifying bacteria in soils is not as high as we expected. It is rather reduced in soils due to the interactions (sorption and complexation) of Ag+/AgNPs with inorganic and organic soil components. This research provides an important viewpoint of the AgNP toxicity to common bacteria in soils. In assessing the impact of AgNPs to microbially mediate soil processes, it might be important to further investigate the interactions and reactivity of AgNPs at the soil-water interface in conjunction with the response to specific microbial communities and the community characterization.

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