Iron promoted reduction of chromate by dissimilatory iron-reducing bacteria.

Chromate is a priority pollutant within the U.S. and many other countries, the hazard of which can be mitigated by reduction to the trivalent form of chromium. Here we elucidate the reduction of Cr(VI) to Cr(III) via a closely coupled, biotic-abiotic reductive pathway under iron-reducing conditions. Injection of chromate into stirred-flow reactors containing Shewanella alga strain BrY and iron (hydr)oxides of varying stabilities results in complete reduction to Cr(III). The maximum sustainable Cr(VI) reduction rate was 5.5 micrograms CrVI.mg-cell-1.h-1 within ferric (hydr)oxide suspensions (surface area 10 m2). In iron limited systems (having HEPES as a buffer), iron was cycled suggesting it acts in a catalytic-type manner for the bacterial reduction of Cr(VI). BrY also reduced Cr(VI) directly; however, the rate of direct (enzymatic) reduction is considerably slower than by Fe(II)(aq) and is inhibited within 20 h due to chromate toxicity. Thus, dissimilatory iron reduction may provide a primary pathway for the sequestration and detoxification of chromate in anaerobic soils and water.

CoIIIEDTA- reduction by Desulfovibrio vulgaris and propagation of reactions involving dissolved sulfide and polysulfides.

The migration of 60Co, dominantly via transport of Co-EDTA complexes, into surface water and groundwater is a recognized concern at many nuclear production and storage sites. Reduction of CoIIIEDTA- to CoIIEDTA2- should decrease the mobility of 60Co in natural environments by stimulating ligand displacement with Fe(III) or Al(III) or by precipitation of CoSx in sulfidic environments. In this study, we examine direct (enzymatic) and indirect (metabolite) reduction processes of CoIIIEDTA- by the sulfate-reducing bacterium Desulfovibrio vulgaris. D. vulgaris reduces CoIIIEDTA- to CoIIEDTA2-, but growth using it as a terminal electron acceptor was not demonstrated. Rather than acting as a competing electron acceptor and limiting cobalt reduction, introducing sulfate with D. vulgaris enhances the reduction of CoIIIEDTA- as a result of sulfide production. Sulfide reduces CoIIIEDTA- in a pathway involving polysulfide formation and leads to a CoS precipitate. Thus, both direct and indirect (i.e., through the production of sulfide) microbial reduction pathways of CoIIIEDTA- may help to retard its migration within soils and waters.

Purification to homogeneity and characterization of a novel Pseudomonas putida chromate reductase.

Cr(VI) (chromate) is a widespread environmental contaminant. Bacterial chromate reductases can convert soluble and toxic chromate to the insoluble and less toxic Cr(III). Bioremediation can therefore be effective in removing chromate from the environment, especially if the bacterial propensity for such removal is enhanced by genetic and biochemical engineering. To clone the chromate reductase-encoding gene, we purified to homogeneity (>600-fold purification) and characterized a novel soluble chromate reductase from Pseudomonas putida, using ammonium sulfate precipitation (55 to 70%), anion-exchange chromatography (DEAE Sepharose CL-6B), chromatofocusing (Polybuffer exchanger 94), and gel filtration (Superose 12 HR 10/30). The enzyme activity was dependent on NADH or NADPH; the temperature and pH optima for chromate reduction were 80 degrees C and 5, respectively; and the K(m) was 374 microM, with a V(max) of 1.72 micromol/min/mg of protein. Sulfate inhibited the enzyme activity noncompetitively. The reductase activity remained virtually unaltered after 30 min of exposure to 50 degrees C; even exposure to higher temperatures did not immediately inactivate the enzyme. X-ray absorption near-edge-structure spectra showed quantitative conversion of chromate to Cr(III) during the enzyme reaction.

Microbiological and geochemical characterization of fluvially deposited sulfidic mine tailings

The fluvial deposition of mine tailings generated from historic mining operations near Butte, Montana, has resulted in substantial surface and shallow groundwater contamination along Silver Bow Creek. Biogeochemical processes in the sediment and underlying hyporheic zone were studied in an attempt to characterize interactions consequential to heavy-metal contamination of shallow groundwater. Sediment cores were extracted and fractionated based on sediment stratification. Subsamples of each fraction were assayed for culturable heterotrophic microbiota, specific microbial guilds involved in metal redox transformations, and both aqueous- and solid-phase geochemistry. Populations of cultivable Fe(III)-reducing bacteria were most prominent in the anoxic, circumneutral pH regions associated with a ferricrete layer or in an oxic zone high in organic carbon and soluble iron. Sulfur- and iron-oxidizing bacteria were distributed in discrete zones throughout the tailings and were often recovered from sections at and below the anoxic groundwater interface. Sulfate-reducing bacteria were also widely distributed in the cores and often occurred in zones overlapping iron and sulfur oxidizers. Sulfate-reducing bacteria were consistently recovered from oxic zones that contained high concentrations of metals in the oxidizable fraction. Altogether, these results suggest a highly varied and complex microbial ecology within a very heterogeneous geochemical environment. Such physical and biological heterogeneity has often been overlooked when remediation strategies for metal contaminated environments are formulated.