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theses

Mercury is a global pollutant that cycles in the environment. Anaerobic bacteria mediate the transformation of mercury which influences its toxicity and distribution. Methylmercury (MeHg) is a form of mercury that is of great concern because it is neurotoxic and bioaccumulates in the food chain. Previous studies focused on understanding the methylation of divalent mercury [Hg(II)], however, little work has been done to study the steps before and after MeHg production. Steps such as the transformation of mercury by pure cultures and microbial communities when elemental mercury [Hg(0)] is the sole or dominant source, the uptake of Hg(II) prior to methylation, the adsorption of MeHg after being released into the environment were still poorly understood. The objective of this study was to understand how anaerobic microorganisms control the formation and transport of MeHg.

Oxidation of Hg(0) is a key step for MeHg production. In Chapter 2, the mechanism of Hg(0) oxidation mediated by the sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 is described. When Hg(0) was provided as the sole mercury source, it diffused into the bacterial cells and was oxidized by cellular thiol functional groups. X-ray absorption spectroscopy results confirmed the formation of oxidized mercury and showed that oxidized mercury was bound to intracellular thiol groups. This work demonstrated an important step of Hg(0) transformation inside the cells that provides substrate for methylation.

The results presented in Chapter 3 demonstrate the importance of MeHg adsorption, which can influence the fate of MeHg in the environment. Geobacter bemidjiensis Bem, a strain of iron-reducing bacteria that was reported to both produce and degrade MeHg was used as a model organism to study MeHg adsorption. X-ray absorption spectroscopy results show that MeHg was bound to the thiol functional groups on the cell membrane. By modeling the adsorption isotherm and the concentration of thiol sites on the cell membrane, the binding constant of MeHg to bacterial cells was estimated. This study highlighted the influence of MeHg adsorption on net MeHg production and provided a parameter that can be used in modeling mercury in the environment.

In Chapter 4, the uptake of inorganic Hg(II) in the mercury methylator Geobacter sulfurreducens PCA and a mutant strain of this species lacking the ability to methylate mercury was studied using mercury stable isotopes. The isotopic composition of Hg(II) was analyzed in different compartments of the bacterial cells. The offset in isotopic composition between Hg(II) in the dissolved phase and Hg(II) inside the cells demonstrates isotope fractionation during Hg(II) uptake. The results of this study provide preliminary data on bioavailable Hg(II) pools utilized by bacteria for MeHg production.

In Chapter 5, results of experimental studies were extended to the real world. The methylation of mercury by microbial communities in estuarine sediments when Hg(0) was provided as the dominant mercury source was studied. By inhibiting the activities of certain families of microorganisms and conducting mercury methylation experiments, methanogens were found to play an important role in producing MeHg. These results reveal that methanogens are an overlooked groups of mercury methylators in estuarine sediments, especially when exposed to Hg(0).

ETD_10271

Ph.D.

Includes bibliographical references

doi:10.7282/t3-121n-7w38

ETD doctoral

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