Isolation of an arsenate-respiring bacterium from a redox front in an arsenic-polluted aquifer in West Bengal, Bengal Basin.

Natural pollution of groundwater by arsenic adversely affects the health of tens of millions of people worldwide, with the deltaic aquifers of SE Asia being particularly polluted. The pollution is caused primarily by, or as a side reaction of, the microbial reduction of sedimentary Fe(III)-oxyhydroxides, but the organism(s) responsible for As release have not been isolated. Here we report the first isolation of a dissimilatory arsenate reducer from sediments of the Bengal Basin in West Bengal. The bacterium, here designated WB3, respires soluble arsenate and couples its reduction to the oxidation of acetate; WB3 is therefore implicated in the process of arsenic pollution of groundwater, which is largely by arsenite. The bacterium WB3 is also capable of reducing dissolved Fe(III) citrate, solid Fe(III)-oxyhydroxide, and elemental sulfur, using acetate as the electron donor. It is a member of the Desulfuromonas genus and possesses a dissimilatory arsenate reductase that was identified using degenerate polymerase chain reaction primers. The sediment from which WB3 was isolated was brown, Pleistocene sand at a depth of 35.2 m below ground level (mbgl). This level was some 3 cm below the boundary between the brown sands and overlying reduced, gray, Holocene aquifer sands. The color boundary is interpreted to be a reduction front that releases As for resorption downflow, yielding a high load of labile As sorbed to the sediment at a depth of 35.8 mbgl and concentrations of As in groundwater that reach >1000 µg/L.

[Influence of transition metal compounds on superoxide dismutase activity of sulfur reducing Desulfuromonas acetoxidans bacteria].

Superoxide dismutase, as one of the enzymes of cells' antioxidant defensive system, catalyzes superoxide anion-radical (O2-) dismutation with O2 and H2O2 forming. The influence of such transition metal compounds, as FeSO4, FeCl3, MnCl2, NiCl2, and CoCl2 on superoxide dismutase activity of sulfur-reducing Desulfuromonas acetoxidans bacteria has been investigated. Maximal activity of the investigated enzyme has been observed accordingly under the influence of 1.0 mM of NiCl2, 2.0 mM of CoCl2 and MnCl2 on the second day and under the influence of 1.0 mM of FeCl3 and FeSO4 respectively, on the third day of growth in comparison with control samples. An increase of incubation time and concentration of metal compound in the medium caused the inhibition of superoxide dismutase activity.

Exploration of the 'cytochromome' of Desulfuromonas acetoxidans, a marine bacterium capable of powering microbial fuel cells.

Recent progress in bacterial genomic analysis has revealed a vast number of genes that encode c-type cytochromes that contain multiple heme cofactors. This high number of multiheme cytochromes in several bacteria has been correlated with their great respiratory flexibility, and in what concerns biotechnological applications, has been correlated with electricity production in Microbial Fuel Cells. Desulfuromonas acetoxidans, a member of the Geobactereaceae family, is one of these organisms for which the genome was recently made available, coding for 47 putative multiheme cytochromes. The growth of D. acetoxidans in different media allowed the identification of the cytochromes dominant in each condition. The triheme cytochrome c(7) is always present suggesting a key role in the bioenergetic metabolism of this organism, and a dodecaheme cytochrome of low homology with other proteins in the databases was also isolated. Different cytochromes are found for different growth conditions showing that their roles can be assigned to specific bioenergetic electron transfer routes.

Dehalorespiration model that incorporates the self-inhibition and biomass inactivation effects of high tetrachloroethene concentrations.

In the vicinity of dense nonaqueous phase liquid (DNAPL) contaminant source zones, aqueous concentrations of tetrachloroethene (PCE) in groundwater may approach saturation levels. In this study, the ability of two PCE-respiring strains (Desulfuromonas michiganensis and Desulfitobacterium strain PCE1) to dechlorinate high concentrations of PCE was experimentally evaluated and depended on the initial biomass concentration. This suggests high PCE concentrations permanently inactivated a fraction of biomass, which, if sufficiently large, prevented dechlorination from proceeding. The toxic effects of PCE were incorporated into a model of dehalorespirer growth by adapting the transformation capacity concept previously applied to describe biomass inactivation by products of cometabolic TCE oxidation. The inactivation growth model was coupled to the Andrews substrate utilization model, which accounts for the self-inhibitory effects of PCE on dechlorination rates, and fit to the experimental data. The importance of incorporating biomass inactivation and self-inhibition effects when modeling reductive dechlorination of high PCE concentrations was demonstrated by comparing the goodness-of-fit of the Andrews biomass inactivation and three alternate models that do capture these factors. The new dehalorespiration model should improve our ability to predict contaminant removal in DNAPL source zones and determine the inoculum size needed to successfully implement bioaugmentation of DNAPL source zones.

Removal of heavy metals and arsenic from contaminated soils using bioremediation and chelant extraction techniques.

A combined chemical and biological treatment scheme was evaluated in this study aiming at obtaining the simultaneous removal of metalloid arsenic and cationic heavy metals from contaminated soils. The treatment involved the use of the iron reducing microorganism Desulfuromonas palmitatis, whose activity was combined with the chelating strength of EDTA. Taking into consideration that soil iron oxides are the main scavengers of As, treatment with iron reducing microorganisms aimed at inducing the reductive dissolution of soil oxides and thus obtaining the release of the retained As. The main objective of using EDTA was the removal of metal contaminants, such as Pb and Zn, through the formation of soluble metal chelates. Experimental results however indicated that EDTA was also indispensable for the biological reduction of Fe(III) oxides. The bacterial activity was found to have a pronounced positive effect on the removal of arsenic, which increased from the value of 35% obtained during the pure chemical treatment up to 90% in the presence of D. palmitatis. In the case of Pb, the major part, i.e. approximately 85%, was removed from soil with purely chemical mechanisms, whereas the biological activity slightly improved the extraction, increasing the final removal up to 90%. Co-treatment had negative effect only for Zn, whose removal was reduced from 80% under abiotic condition to approximately 50% in the presence of bacteria.

Mercury methylation by dissimilatory iron-reducing bacteria.

The Hg-methylating ability of dissimilatory iron-reducing bacteria in the genera Geobacter, Desulfuromonas, and Shewanella was examined. All of the Geobacter and Desulfuromonas strains tested methylated mercury while reducing Fe(III), nitrate, or fumarate. In contrast, none of the Shewanella strains produced methylmercury at higher levels than abiotic controls under similar culture conditions. Geobacter and Desulfuromonas are closely related to known Hg-methylating sulfate-reducing bacteria within the Deltaproteobacteria.