Carbon microbial reduction

Workman SL Woods, YA Gorby, JK Fredrickson, and MJ Trnex (1997) Microbial reduction of vitamin B,2 by Shewanella alga strain BrY with snbseqnent transformation of carbon tetrachloride. Environ Sci Technol 31 2292-2297. 

Nealson KH, CR Myers (1992) Microbial reduction of manganese and iron new approaches to carbon cycling. Appl Environ Microbiol 58 439-443. 

There are two ways by which goethite can be formed in soils. If iron is released from solid Fe" compounds such as Fe silicates, carbonates and sulphides or, alternatively, from existing Fe" oxides by microbial reduction, the Fe will be oxidized in an... 

Scheme 15)62. After terminating the reaction at a conversion of 38% (relative to total amount of substrate rac-78), the product (S)-43 was separated from the nonreacted substrate by column chromatography on silica gel and isolated on a preparative scale in 71% yield (relative to total amount of converted rac-78) with an enantiomeric purity of 95% ee. Recrystallization led to an improvement of the enantiomeric purity by up to >98% ee. The biotransformation product (S)-43 is the antipode of compound (/ )-43 which was obtained by enantioselective microbial reduction of the acylsilane 42 (see Scheme 8)53. The nonreacted substrate (/ )-78 was isolated in 81% yield (relative to total amount of nonconverted rac-78) with an enantiomeric purity of 57% ee. For further enantioselective enzymatic hydrolyses of racemic organosilicon esters, with the carbon atom as the center of chirality, see References 63 and 64. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Sulfate retention in softwater lakes appears to increase in proportion to sulfate loadings (IQ). This raises the issue of factors that limit the magnitude of sulfate retention. At least three potential conditions might limit sulfate reduction rates 1) supply of Fe2+ to sequester reduced S, 2) supply of carbon to support microbial reduction, and 3) inhibition of sulfate reauction by acidification. 

Since we are interested in the sulfur-carbon chemistry of diagenesis, the relevant geochemical processes are limited to those occurring within the very narrow temperature span of approximately 0 to 60 °C. The reactions must occur under hydrous conditions although in media having at least some capacity to accommodate lipophilic source molecules as a result of the presence of substances such as low grade carbohydrates and small carboxylic acids or their salts. If the sulfur source is a form of reduced sulfur from active microbial reduction of sulfate, the pH will range from near neutral... 

Anaerobic organisms are able to use oxidized chemical species such as nitrate (NO ) as electron acceptors in place of molecular oxygen. Consequently, microbial reduction of nitrate to N2 (a process termed denitrification ) occurs in the early stages of soil reduction, as does Mn oxide reduction to Mn. Reduced species such as nitrite (NO ) and Mn then appear in solution. As more extreme reducing conditions develop, ammonium (NH ) accumulates from nitrogen reduction reactions, and iron solubility increases in the form of Fe. The elevated iron and manganese solubility is ultimately limited by precipitation of the rather insoluble carbonates of Fe (siderite) and Mn (rhodochrosite) if the soil pH is not too low ... 

Better results were obtained by the microbial reduction of 6-chloropyridine 3-carboxylate using carbon monoxide. The product/by-product ratio was about 80, if a carbon monoxide pressure of 20 bar was applied. In another experiment ethylene glycol diethyl ether was used as a second phase in order to transfer the product to the organic solvent (Table 20) (86). 

For the enantioselective preparations of chiral synthons, the most interesting oxidations are the hydroxylations of unactivated saturated carbons or carbon-carbon double bonds in alkene and arene systems, together with the oxidative transformations of various chemical functions. Of special interest is the enzymatic generation of enantiopure epoxides. This can be achieved by epoxidation of double bonds with cytochrome P450 mono-oxygenases, w-hydroxylases, or biotransformation with whole micro-organisms. Alternative approaches include the microbial reduction of a-haloketones, or the use of haloperoxi-dases and halohydrine epoxidases [128]. The enantioselective hydrolysis of several types of epoxides can be achieved with epoxide hydrolases (a relatively new class of enzymes). These enzymes give access to enantiopure epoxides and chiral diols by enantioselective hydrolysis of racemic epoxides or by stereoselective hydrolysis of meso-epoxides [128,129]. 

This concept of sulphur fractionation in soils has been successfully applied to sewage sludge (Sommers et al., 1977) and to freshwater and marine derived peat-forming systems (Cas ande et al., 1977). In the marine peat, carbon-bonded sulphiu" accounted for 50% of total sulphur while ester sulphate constituted only ca. 25%. These authors noted an overall increase in sulphur in going from plant samples to peat in the marine environment, and concluded that plants were not the dominant sulphur-concentrating mechanism. The sulphur was probably delivered to a large extent by sulphate diffusion and microbial reduction, whereby carbon-bonded sulphur acted as a sink for sulphur in the peat. 

Other energy-producing reactions of organisms involve the reduction of oxygen to water, the reduction of nitrate to ammonia and nitrogen gas, the reduction of sulfate to sulfide, and the reduction of carbon dioxide to methane. All of these reactions can exert a profound effect on water quality especially when it is realized that the affected chemical species also engage in many other chemical reactions. For example, the sulfide ion forms precipitates with many heavy metals. The microbial reduction of sulfate to sulfide could be accompanied by a reduction in the dissolved heavy metal content in a natural water. 

A role of microbial processes in release of arsenic into groundwater concomitant with the reductive dissolution of Fe(ni) oxyhydroxides has been suggested based on the observed correlation between dissolved arsenic and bicarbonate concentrations (94,95). Increased bicarbonate concentrations are attributed to the oxidation of organic matter with Fe(III) oxyhydroxides as the terminal electron acceptor. Like oxidative dissolution, reductive dissolution may be kinetically limited. Rates of microbial reduction may be limited by the supply (and nature) of organic carbon. 

The reduction of arsenate to arsenite by sulfide ion (expected in the anaerobic digester) is often accompanied by the formation of elemental sulfur and/or polysulfides represented as HS2 in Eq. (3) (Helz et al, 1995 Rochette et al, 2000). Microbial reduction of arsenate and arsenite under anaerobic conditions is well documented (Macur et al, 2001) and may result in the formation of arseno-carbonate complexes ... 

Denitrification (often referred to as dissimilatory nitrate reduction) is the microbial reduction of NO3" to N2. The reaction steps require an organic carbon as electron donor (CH2O generically used) and are shown below ... 

Boon, N., Fan, M.Z., Zhang, L, and Zhang, X.Y. (2009) A completely anoxic microbial fuel cell using a photo-biocathode for cathodic carbon dioxide reduction. Energy Environ. Sci.,... 

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