Dissimilatory reduction, anaerobic bacteria

Organic Carbon Oxidation Microbially Coupled to Reduction of Fe(III)(hydr)oxide. More and more evidence is accumulating that bacteria can grow anaerobically by coupling organic carbon oxidation to the dissimilatory reduction of iron(III) oxides (Nealson, 1982 Arnold et al., 1986 Lovely and Philips, 1988 Nealson and Myers, 1990). 

Dissimilatory reduction by anaerobic bacteria occurs in the anoxic hy-polimnion of stratified lakes and in sediments just below the oxic-anoxic boundary. It produces H2S,... 

Sulfate Reduction. Dissimilatory sulfate reduction, anaerobic respiration with sulfate as the terminal electron acceptor, is performed by relatively few genera of bacteria (84). Many bacteria and algae are able to... 

As noted in Section 62.1.9.6, reduction of nitrate may occur by assimilatory or dissimilatory pathways. In the former case, the nitrate produced is reduced further to ammonia, which is incorporated into the cell. In the latter case, nitrate is reduced anaerobically to nitrite, serving as an electron acceptor in the respiration of facultative or a few obligate anaerobic bacteria. The example of Escherichia coli has been considered in Section 62.1.13.4.3. This process is usually terminated at nitrite, which accumulates around the cells, but may proceed further1511 as nitrite-linked respiration in the process of denitrification. 

When obligate anaerobic bacteria carry out dissimilatory sulfate reduction, they are referred to as sulfate reducers or sulfidogens. The traditional sulfate-reducing genera are Desulfovibrio and Desulfotomaculum. Sulfate reduction results in the production of hydrogen sulfide ... 

There are two pathways of dissimilatory nitrate reduction, generally thought to be mediated by anaerobic, or facultatively anaerobic bacteria, using NOs" as a terminal electron acceptor in respiration (Fig. 21.ID and F) (see Chapter 6, Devol, this volume). One pathway leads to production of ammonium, and may act as an internal cychng loop within the system (D Elia and Wiebe, 1990). The other pathway, denitrification, ends in production of N2O and/or N2 gas, which can then be lost from the system to the atmosphere. 

Table 2 Approximate Time Scales of Microbiologically Mediated Reduction of Arsenate to Arsenite Performed by Bacteria Capable of Utilizing As(V) as a Terminal Electron Acceptor During Anaerobic Respiration (Dissimilatory Reduction) or by Organisms Either Containing, or Presumed to Contain, ars Genes that Code for an Arsenate Reductase and an Arsenite Efflux Pump...

When particulate preparations from bacteria grown anaerobically in the presence of nitrate are incubated with respiratory substrates or reduced pyridine nucleotides, it is observed that the cytochromes become reduced (Knook et ai, 1973 Lam and Nicholas, 1969 Ruiz-Herrera and DeMoss, 1969 Vila et al., 1977). Because the cytochrome b component is reoxidized when nitrate is added, it appears the Mo-protein (nitrate reductase) transfers electrons from cytochrome b to nitrate. These observations on dissimilatory nitrate reduction by bacteria are summarized in the following scheme. 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

In contrast, many gram-negative bacteria contain a nitrate reductase (EC 1.7.99.4 and/or 1.9.6.1) that also reduces nitrate to nitrite although under anaerobic conditions. The dissimilatory nitrite reduction leading to denitrification encompasses then the reduction of nitrite to nitric oxide by dissimilatory nitrite reductases (NiR, EC 1.7.2.1) that, in combination with nitric oxide reductases (NOR) and nitrous oxide reductases (N2OR), transform nitrite into nitrogen ... 

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