Formation by microbial reduction

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

Internal Sources and Atmospheric Exchange of Methane. Methane is produced by specialized groups of obligate anaerobic bacteria (22, 23). The formation of methane as a metabolic product results either from the microbial reduction of CO2 with molecular H2, or via the fermentation of acetic acid. More structurally complex substrates may also serve as electron acceptors/donors, but the end result of methanogenesis is to produce methane and CO2 as end products (23). 

With strains of Saccharomyces the (25,35)-isomer is produced predominantly, accompanied by some of the (2/J,3,S )-isomer. Similar to ft-keto esters, the formation of the undcsired isomer can be decreased by the addition of an a,/i-unsaturatcd carbonyl compound, e.g.. methyl vinyl ketone, or an allylic alcohol. These additives probably act as inhibitors for the enzyme which produces the (2/ ,35)-isomer202 203. More recently, the microbial reduction of a variety of simple 2,2-disubstituted cyclic 1,3-diketones of various ring size has been investigated204 205 206. In most cases one of the substituents in the 2-position is methyl. The configuration of the hydroxy group in the reduced product is always S, and the enantiomeric excess is often high (Table 7). 

Microbial reduction of toxic organics is carried out by reduction enzymes. Major reduction reactions of selected toxic organics are shown in Table 13.5. The reduction of the nitro group to amine involves the formation of a nitro and a hydroxyamino group. This type of reduction reaction occurs during the microbial metabolism of various pesticides. Organophosphorous pesticides such as para-thion, paraoxon, or fenitrothion are often reduced to nontoxic amino compounds (Miyamoto et al., 1966 Matsumura and Benezet, 1978). 

The formation of arsines, a volatile form of arsenic, is known to occur via microbial activity (27). Soil microbes produce volatile arsenicals by a reductive methyl-ation pathway from inorganic and methylated forms of arsenic (29). Methylation in which a methyl ion is added to an arsenite ion is strictly a biological process... 

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 ... 

Diastereoselective Reduction of Ketones by Baker s Yeast. Asymmetric microbial reduction of oc-substituted ketones leads to the formation of diastereomeilc syn-and anh -products. Because the chiral center on the a-position of the ketone is stereochemically labile, rapid in-situ racemization of the substrate enantiomers occurs via enolization ° - leading to dynamic resolution [67, 895, 896]. Thus, the ratio between the diastereomeric syn- and anti-products is not 1 1, but is determined by the selectivities of the enzymes involved in the reduction process [897]. Under optimal conditions it can even be as high as 100 0 [898]. 

---