A biofilm-forming strain of sulfate-reducing bacteria (SRB), isolated from a naturally occurring mixed biofilm and identified by 16S rDNA analysis as a strain of Desulfomicrobium norvegicum, rapidly removed 200 M selenite from solution during growth on lactate and sulfate. Elemental selenium and elemental sulfur were precipitated outside SRB cells. Precipitation occurred by an abiotic reaction with bacterially generated sulfide. This appears to be a generalized ability among SRB, arising from dissimilatory sulfide biogenesis, and can take place under low redox conditions and in the dark. The reaction represents a new means for the deposition of elemental sulfur by SRB under such conditions. A combination of transmission electron microscopy, environmental scanning electron microscopy, and cryostage field emission scanning electron microscopy were used to reveal the hydrated nature of SRB biofilms and to investigate the location of deposited sulfur-selenium in relation to biofilm elements. When pregrown SRB biofilms were exposed to a selenite-containing medium, nanometer-sized selenium-sulfur granules were precipitated within the biofilm matrix. Selenite was therefore shown to pass through the biofilm matrix before reacting with bacterially generated sulfide. This constitutes an efficient method for the removal of toxic concentrations of selenite from solution. Implications for environmental cycling and the fate of sulfur and selenium are discussed, and a general model for the potential action of SRB in selenium transformations is presented.Sulfate-reducing bacteria (SRB) are a phylogenetically and physiologically diverse group of bacteria, characterized by their common capacity to conserve energy for growth by linking the oxidation of various substrates to the dissimilatory reduction of sulfate (S 6ϩ ) to sulfide (S 2Ϫ ). As such, SRB comprise a functional group within a sulfuretum, linking broad-scale cycling between sulfate and sulfide by ecological communities of SRB and sulfide-oxidizing bacteria (12, 28). Biological reoxidation of reduced sulfur species typically occurs at oxic-anoxic transition zones and is attributed largely to phototrophs and chemolithotrophs. Small-scale cycling through elemental sulfur (S 0 ) also occurs and is generally attributed to syntrophic associations of sulfide oxidizers and sulfur reducers (3,17).The biological cycling of selenium is receiving increasing attention, due not only to the biological importance of selenium as an essential trace element but also to the potential for selenium pollution to cause significant ecological damage (42). Selenium is a group 16 metalloid element possessing several stable oxidation states. Under oxic conditions, selenium is present mostly as the oxyanions selenite (SeO 3 2Ϫ , Se 4ϩ oxidation state) and selenate (SeO 4 2Ϫ , Se 6ϩ oxidation state), whereas under anoxic conditions, selenide (Se 2Ϫ ) and elemental selenium (Se 0 ) appear predominant (5, 6). Selenium is incorporated by organisms through selenide, is important in some enzyme systems,...
A biofilm-selected strain of a Desulfomicrobium sp. removed selenate from solution to sub-micromolar concentrations during growth on lactate (or hydrogen) and sulfate. Under sulfate-limited growth conditions, selenium was enzymatically reduced to selenide. Under excess sulfate conditions, selenate removal was primarily by enzymatic reduction to elemental selenium. Sequestration by biofilms was greater under the latter condition. Experiments with washed cell suspensions showed that high sulfate concentrations inhibited cell-specific selenate reduction, but when growing cells were exposed to selenate, the biomass increase achieved during incubations with abundant sulfate resulted in more rapid selenate removal. The addition of small amounts of sulfite, or thiosulfate, ameliorated this inhibition. Nitrate also inhibited selenate reduction in washed cell suspensions, apparently due to a general oxidizing effect. These results suggest that where biofilm-based sulfate-reducing bacteria (SRB) bioreactors are considered for the treatment of mixed metalliferous wastes that contain selenium oxyanions, adequate selenate removal should be achievable under a range of environmental conditions. The form and fate of the precipitated product will, however, be influenced by the dominant reduction pathway, which is controlled by environmental variables.
A sulphate-reducing consortium used in a bioprocess to remove toxic metals from solution as insoluble sulphides, was characterised using molecular (PCR-based) and traditional culturing techniques. After prolonged cultivation under anoxic biofilm-forming conditions, the mixed culture contained a low diversity of sulphate-reducing bacteria, dominated by one strain closely related to Desulfomicrobium norvegicum, identified by three independent PCR-based analyses. The genetic targets used were the 16S rRNA gene, the 16S-23S rRNA gene intergenic spacer region and the disulfite reductase (dsr) gene, which is conserved amongst all known sulphate-reducing bacteria. This organism was also isolated by conventional anaerobic techniques, confirming its presence in the mixed culture. A surprising diversity of other non-sulphate-reducing facultative and obligate anaerobes were detected, supporting a model of the symbiotic/commensal nature of carbon and energy fluxes in such a mixed culture while suggesting the physiological capacity for a wide range of biotransformations by this stable microbial consortium.
This paper explores the importance of spatial distribution of soil components in terms of the treatment of contaminated soil using bioremediation. Initially described is the spatial distribution of contaminants within typical contaminated sites. Also examined is how a successful bioremedial strategy seeks to overcome problems imposed by heterogeneity and engineer conditions that facilitate contaminant treatment. The spatial distribution of the contaminant in relation to the physicochemical environment and the microflora is examined both at a macro- and micro- scale.
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