Shewanella oneidensis MR-1 is a gram-negative facultative anaerobe capable of utilizing a broad range of electron acceptors, including several solid substrates. S. oneidensis MR-1 can reduce Mn(IV) and Fe(III) oxides and can produce current in microbial fuel cells. The mechanisms that are employed by S. oneidensis MR-1 to execute these processes have not yet been fully elucidated. Several different S. oneidensis MR-1 deletion mutants were generated and tested for current production and metal oxide reduction. The results showed that a few key cytochromes play a role in all of the processes but that their degrees of participation in each process are very different. Overall, these data suggest a very complex picture of electron transfer to solid and soluble substrates by S. oneidensis MR-1.
The research is designed to evaluate the impact of metal-reducing bacteria on the release of radionuclides, specifically uranium and plutonium, from iron hydroxide minerals formed on the surfaces of corroding mild and stainless steels. The ultimate goal is to develop a safe and effective biological approach for decontaminating mild and stainless steels that were used in the production, transport, and storage of radioactive materials.
The reduction kinetics of Fe(III)citrate, Fe(III)NTA, Co(III)EDTA-, U(VI)O(2) (2+), Cr(VI)O(4) (2-), and Tc(VII)O(4) (-) were studied in cultures of dissimilatory metal reducing bacteria (DMRB): Shewanella alga strain BrY, Shewanella putrefaciens strain CN32, Shewanella oneidensis strain MR-1, and Geobacter metallireducens strain GS-15. Reduction rates were metal specific with the following rate trend: Fe(III)citrate > or = Fe(III)NTA > Co(III)EDTA- >> UO(2)(2+) > CrO(4)(2-) > TcO(4)(-), except for CrO(4) (2-) when H(2) was used as electron donor. The metal reduction rates were also electron donor dependent with faster rates observed for H(2) than lactate- for all Shewanella species despite higher initial lactate (10 mM) than H2 (0.48 mM). The bioreduction of CrO(4) (2-) was anomalously slower compared to the other metals with H(2) as an electron donor relative to lactate and reduction ceased before all the CrO(4)(2-) had been reduced. Transmission electron microscopic (TEM) and energy-dispersive spectroscopic (EDS) analyses performed on selected solids at experiment termination found precipitates of reduced U and Tc in association with the outer cell membrane and in the periplasm of the bacteria. The kinetic rates of metal reduction were correlated with the precipitation of reduced metal phases and their causal relationship discussed. The experimental rate data were well described by a Monod kinetic expression with respect to the electron acceptor for all metals except CrO(4)(2-), for which the Monod model had to be modified to account for incomplete reduction. However, the Monod models became statistically over-parameterized, resulting in large uncertainties of their parameters. A first-order approximation to the Monod model also effectively described the experimental results, but the rate coefficients exhibited far less uncertainty. The more precise rate coefficients of the first-order model provided a better means than the Monod parameters, to quantitatively compare the reduction rates between metals, electron donors, and DMRB species.
Yellowstone Lake (Yellowstone National Park, WY, USA) is a large high-altitude (2200 m), fresh-water lake, which straddles an extensive caldera and is the center of significant geothermal activity. The primary goal of this interdisciplinary study was to evaluate the microbial populations inhabiting thermal vent communities in Yellowstone Lake using 16S rRNA gene and random metagenome sequencing, and to determine how geochemical attributes of vent waters influence the distribution of specific microorganisms and their metabolic potential. Thermal vent waters and associated microbial biomass were sampled during two field seasons (2007-2008) using a remotely operated vehicle (ROV). Sublacustrine thermal vent waters (circa 50-90°C) contained elevated concentrations of numerous constituents associated with geothermal activity including dissolved hydrogen, sulfide, methane and carbon dioxide. Microorganisms associated with sulfur-rich filamentous "streamer" communities of Inflated Plain and West Thumb (pH range 5-6) were dominated by bacteria from the Aquificales, but also contained thermophilic archaea from the Crenarchaeota and Euryarchaeota. Novel groups of methanogens and members of the Korarchaeota were observed in vents from West Thumb and Elliot's Crater (pH 5-6). Conversely, metagenome sequence from Mary Bay vent sediments did not yield large assemblies, and contained diverse thermophilic and nonthermophilic bacterial relatives. Analysis of functional genes associated with the major vent populations indicated a direct linkage to high concentrations of carbon dioxide, reduced sulfur (sulfide and/or elemental S), hydrogen and methane in the deep thermal ecosystems. Our observations show that sublacustrine thermal vents in Yellowstone Lake support novel thermophilic communities, which contain microorganisms with functional attributes not found to date in terrestrial geothermal systems of YNP.
The bacterium Shewanella oneidensis MR-1 is a dissimilatory metal-reducing bacterium capable of performing anaerobic respiration using a metal as terminal electron acceptor. Isolated from Lake Oneida in Upstate New York, S. oneidensis MR-1 was first noted for its manganese-reducing capability, but has now been shown to reduce a range of metal ions such as Fe(III), Mn(IV), As(V) and Cr(VI), as well as sulfur anions such as thiosulfate and sulfite. In the lab, Shewanella has been grown anaerobically in media enhanced with sulfur and metal ions in order to produce several types of chalcogenide nanoparticles, such as zinc sulfide and arsenic trisulfide.Given the utility of chalcogenide materials for electronics and photonics applications, bacterially-synthesized chalcogenide nanoparticles present a tantalizing avenue for green chemistry. Compared to similar materials produced using traditional chemical synthesis methods, bacterially-synthesized nanomaterials can be produced at much lower temperatures and using fewer chemical reagents. This work presents a method of synthesizing molybdenum disulfide nanoparticles using S. oneidensis bacteria. Molybdenum disulfide is a layered semiconductor with an indirect band gap in its bulk state and a direct bandgap in its monolayer state. It also exhibits changes in its electronic properties when its surfaces are functionalized with molecules, giving it applications for both photodetection and biosensing. An anaerobic batch culture of S. oneidensis MR-1 was incubated at room temperature in the presence of molybdenum oxide, resulting in the production of molybdenum disulfide crystals less than a micron in diameter. These crystals were detected using scanning electron microscopy, transmission electron microscopy, absorbance spectroscopy and X-ray diffraction. In addition to confirming that molybdenum disulfide can be produced by Shewanella bacteria, the data collected using these methods provide insight on the size, morphology and photoresponse of nanoparticles generated this way. The findings also allow inferences to be made about how the confluence of several mechanisms present in an anaerobic Shewanella culture combine to make such a synthesis possible, while providing clues about how such processes can be further improved or extended to other materials.
