Reduction of uranium(VI) phosphate during growth of the thermophilic bacterium Thermoterrabacterium ferrireducens.

The thermophilic, gram-positive bacterium Thermoterrabacterium ferrireducens coupled organotrophic growth to the reduction of sparingly soluble U(VI) phosphate. X-ray powder diffraction and X-ray absorption spectroscopy analysis identified the electron acceptor in a defined medium as U(VI) phosphate [uramphite; (NH4)(UO2)(PO4) . 3H2O], while the U(IV)-containing precipitate formed during bacterial growth was identified as ningyoite [CaU(PO4)2 . H2O]. This is the first report of microbial reduction of a largely insoluble U(VI) compound.

Vanadate reduction by molybdenum-free dissimilatory nitrate reductases from vanadate-reducing bacteria.

Molybdenum- and molybdenum cofactor-free nitrate reductases recently isolated by us from vanadate-reducing bacteria Pseudomonas isachenkovii are likely to mediate vanadate reduction. During anaerobic growth of P. isachenkovii on medium supplemented with nitrate and vanadate, vanadate dissimilation was followed by nitrate consumption, and this process was associated with some structural reorganizations of nitrate reductases. The homogeneous membrane-bound nitrate reductase of P. isachenkovii reduced vanadate with NADH as an electron donor.

Vanadium-binding protein excreted by vanadate-reducing bacteria.

A vanadium-binding protein was isolated from the culture medium of the vanadium-reducing bacterium Pseudomonas isachenkovii by utilizing vanadate as the terminal electron acceptor upon anaerobic respiration. The protein was associated with vanadium at a molar ratio of approximately 1:20. It was purified to homogeneity and separated into three components by treatment with 1 M HCl followed by gel filtration: a protein, a vanadium-binding ligand, and inorganic vanadium. Electron paramagnetic resonance analysis showed that vanadium was associated with the protein in the 4+ oxidation state. The distribution of vanadium within the cell was studied by electron microscopy and x-ray microanalysis of P. isachenkovii cells. The results suggest that vanadium, accumulated in special swells on the surface of the cell membranes, is reduced and excreted to the medium.

Reduction of technetium by Desulfovibrio desulfuricans: biocatalyst characterization and use in a flowthrough bioreactor.

Resting cells of Desulfovibrio desulfuricans coupled the oxidation of a range of electron donors to Tc(VII) reduction. The reduced technetium was precipitated as an insoluble low-valence oxide. The optimum electron donor for the biotransformation was hydrogen, although rapid rates of reduction were also supported when formate or pyruvate was supplied to the cells. Technetium reduction was less efficient when the growth substrates lactate and ethanol were supplied as electron donors, while glycerol, succinate, acetate, and methanol supported negligible reduction. Enzyme activity was stable for several weeks and was insensitive to oxygen. Transmission electron microscopy showed that the radionuclide was precipitated at the periphery of the cell. Cells poisoned with Cu(II), which is selective for periplasmic but not cytoplasmic hydrogenases, were unable to reduce Tc(VII), a result consistent with the involvement of a periplasmic hydrogenase in Tc(VII) reduction. Resting cells, immobilized in a flowthrough membrane bioreactor and supplied with Tc(VII)-supplemented solution, accumulated substantial quantities of the radionuclide when formate was supplied as the electron donor, indicating the potential of this organism as a biocatalyst to treat Tc-contaminated wastewaters.

Molybdenum-free nitrate reductases from vanadate-reducing bacteria.

Two catalytically distinct molybdenum-free dissimilatory nitrate reductases, a soluble periplasmic and a membrane-bound one, were isolated from the vanadate-reducing facultatively anaerobic bacterium Pseudomonas isachenkovii and purified to electrophoretic homogeneity. The enzymes did not contain molybdenum, the periplasmic enzyme contained vanadium, whereas the membrane-bound enzyme was vanadium-free. Both nitrate reductases lacked molybdenum cofactor. This fact was proved by reconstitution of the apoprotein of the nitrate reductase of Neurospora crassa nit-1 mutant. This is the first demonstration of molybdenum-free and molybdenum cofactor-free nitrate reductases.