Microbial Transformation And Interactions With Uranium And Mercury In The Environment

Complex interactions between microbes and radionuclides or heavy metals include sorption and uptake, complexation and precipitation, reduction and oxidation, methylation and demethylation. These complex biogeochemical interactions affect the chemical speciation, transfromation, transport and cycling of radionuclides and heavy metals in the environment. Understanding these complex interactions is critically important to better assess the environmental consequences and better design remediation strategies for these high-priority pollutants. This dissertation focuses on the following three chapters.I. Biosorption mechanisms of uranium(VI) by Saccharomyces cerevisiae under environmentally relevant conditions. Biosorption is of significance for the safety evaluation of high-level nuclear waste repositories and remediation of radioactive contamination sites. Quantitive study and structural characterization of uranium sorbed by both live and heat-killed Saccharomyces cerevisiae were carried out at environmentally relevant uranium concentrations and with different ionic strengths. Kinetic investigation showed that the reaction reached an equilibrium within 15 min. In equilibrium studies, pH shifted towards neutral due to the release of hydroxyl ions. pH was found to be the most important factor, affecting electrostatic interactions between uranyl ions and S. cerevisiae surface. The high ionic strength inhibited biosorption capacity, which can be explained by a competitive reaction between sodium ions and uranyl ions. Heat killing process significantly enhanced biosorption capacity, showing an order of magnitude higher sorption than that of live cells. High resolution transmission electron microscopy(HRTEM) coupled with energy dispersive X-ray(EDX) analysis showed needle-like uranium-phosphate precipitation formed on the cell walls for both live and heat-killed cells. Besides, dark-field micrographs displayed considerable similar uranium-phosphate precipitation presented outside the heat-killed cells. FTIR illustrated function groups such as hydroxyl, carboxyl, phosphate, and amine played important roles in complexation with uranium.II. Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem. Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury(MeHg) in natural ecosystems. Although mercury(Hg) methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium Geobacter bemidjiensis Bem. Here we report, for the first time, that the strain G. bemidjiensis Bem can mediate a suite of Hg transformations, including Hg(II) reduction, Hg(0) oxidation, MeHg production and degradation under anoxic conditions. Results suggest that G. bemidjiensis utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologues of a organomercurial lyase(MerB) and a mercuric reductase(MerA). In addition, the cells can strongly sorb Hg(II) and MeHg, reduce or oxidize Hg, resulting in both time and concentration-dependent Hg species transformations. Moderate concentrations(10–500 mM) of Hg-binding ligands such as cysteine enhance Hg(II) methylation but inhibit MeHg degradation. Addition of naturally dissolved organic matter(DOM) is found to compete with cells for Hg binding and inhibit both Hg(II) methylation and MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments.III. Methylmercury uptake and degradation by methanotrophs Methylosinus trichosporium OB3 b and Methylococcus capsulatus Bath. While both of Hg methylation and MeHg degradation have been investigated extensively, no studies have examined MeHg degradation by methanotrophs, despite their ubiquitous presence in the environment. Here, we report that methanotrophs such as Methylosinus trichosporium OB3 b can take up and degrade MeHg rapidly, whereas others such as Methylococcus capsulatus Bath can only take up but cannot degrade MeHg. M. trichosporium OB3 b cells grown in the presence of 1 mM copper ions(Cu2+) degraded less MeHg but degraded more MeHg in the presence of 25 mM cerium(Ce3+) than those grown in the absence of metals. Importantly, demethylation rates by OB3 b was found to increase with increasing MeHg concentrations, but addition of methanol(0.1% v/v) completely inhibited MeHg degradation. These results suggest that M. trichosporium OB3 b likely utilized MeHg as a C1-carbon and energy source in the absence of methane. This study thus demonstrates a previously unrecognized pathway of MeHg degradation by methanotrouphs and possible broader involvement of C1-metabolizing aerobes in the degradation and cycling of toxic MeHg in the environment. These findings are critically important to further understand microbial transformations of mercury in the environment and its bioaccumulation in the food chain.