Microbial oxidation, species-specific

Subterminal alkane oxidation apparently occurs in some bacterial species (Markovetz, 1971). This type of oxidation is probably responsible for the formation of long-chain secondary alcohols and ketones. Pirnik (1977) and Perry (1984) have reviewed the microbial oxidation of branched and cyclic alkanes, respectively. Interestingly, none of the cyclohexane or cyclopentane compounds seems to be metabolized by pure cultures. Rather, non-specific oxidases present in many bacteria convert the cyclic alkanes into cyclic ketones, which are then oxidized by specific bacteria. 

Bacteria may catalyze and considerably enhance the oxidation of pyrite and Fe(II) in water, especially under acidic conditions (Welch et al., 2000, 597). Many microbial species actually oxidize only specific elements in sulfides. With pyrite, Acidithiobacillus thiooxidans is important in the oxidation of sulfur, whereas Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans (formerly Thiobacillus fer-rooxidans) oxidize Fe(II) (Gleisner and Herbert, 2002, 140). Acidithiobacillus ferrooxidans obtain energy through Reaction 3.45 (Gleisner and Herbert, 2002, 140). The bacteria are most active at about 30 °C and pH 2-3 (Savage, Bird and Ashley, 2000, 407). Acidithiobacillus sp. and Leptospirillum ferrooxidans have the ability to increase the oxidation of sulfide minerals by about five orders of magnitude (Welch et al., 2000, 597). 

Transformation by a single microbial species Some geomicrobial transformations in nature involve a single microbial species. An example of such a transformation is the anaerobic reduction of a Mn(IV) oxide to Mn " " by S. oneidensis or G. metallireducens in an environment with a plentiful supply of an appropriate electron donor like lactate for S. oneidensis or acetate for G. metallireducens. Because each of these two microbial species can perform the reduction by themselves, and because electron donors like lactate and acetate are formed as major end-products in the energy metabolism of a variety of microbes present in the same environment that harbours S. oneidensis or G. metallireducens, the latter do not need to form specific microbial associations to bring about Mn(IV) oxide reduction. 

The other strategy mentioned above is the biosynthesis, isolation and purification of individual flavor-active species. This approach involves exploiting specific bio-conversions, such as oxidation and reduction or de novo syntheses by either microbial fermentation or by using specific enzyme systems. 

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