Dissimilatory Reduction of Nitrate

Cole, J. A., 1988, Assimilatory and dissimilatory reduction of nitrate to ammonia, Symp. Soc. Gen. Microbiol. 42 2819329. 

Binnerup, S.J., Jensen, K., Revsbech, N. P., Jensen, M. H., and Sorensen, J. (1992). Denitrification, dissimilatory reduction of nitrate to ammonium, and nitrification in a bioturbated estuarine sediment as measured with N-15 and microsensor techniques. Applied and Environmental Microbiology 58, 303-313. 

Dissimilatory Reduction of Nitrate to Ammonium by Microbial Cultures. We studied nitrate reduction to ammonia by an obligate anaerobe, Clostridium, which cannot gain energy from this reduction by electron transport phosphorylation, and by a number of Enterobacteri-aceae (known to be nitrate respirers) that can gain energy via the nitrate to nitrite step. All these organisms converted NOg" to as the... 

Dissimilatory reduction of nitrate to ammonia is performed by obligate and facultative anaerobes with fermentative metabolism, including Clostridium and Bacillus species (Tiedje, 1988). These organisms, in contrast to denitrifiers, usually do not rely on nitrate as electron acceptor. Therefore, DNRA involves 8e transfer as compared to 5e transfer for denitrification, suggesting that more organic substrate can be potentially degraded by DNRA. However, nitrate availability under DNRA conditions is usually very low because much of the nitrate formed during nitrification under aerobic conditions is rapidly consumed by denitrifiers in adjacent anaerobic environments. 

Soils inclnding wetland soils are important sonrces of atmospheric nitrous oxide. A wide range of processes may produce nitrous oxide, as well as minor amounts of NO, but not all of these seem to be fully understood. The main biological processes of nitrous oxide formation in soils are shown in Figure 16.5. They include nitrification, denitrification, the dissimilatory reduction of nitrate to ammonium, and the assimilatory reduction of nitrate wherein N is incorporated in the cell biomass. Additionally, some NO and nitrous oxide may be released due to chemo-denitrification and pyro-denitrification. Of these processes, nitrification and denitrification are the most important with respect to nitrous oxide production. 

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also) into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety of microorganisms in the forest ecosystems. Measurable rates of N20 production have been observed in many forest soils. The values from 2.1 to 4.0 kg/ha/yr are typical for forest soils in various places of Boreal and Sub-Boreal Forest ecosystems. All in situ studies (field monitoring) of denitrification in forest soils have shown large spatial and temporal variability in response to varying soils characteristics such as acidity, temperature, moisture, oxygen, ambient nitrate and available carbon. 

The fact that the cytochrome P-450 was induced even in the presence of NH3, which is the end product of assimilatory N-oxide reductions, suggested that it might funciton in dissimilatory N-oxide reductions. Anaerobic growth experiments with induced cells showed that reduction of nitrate to nitrite was energy yielding in F. oxysporum but reduction of nitrite to N2O was probably not (Shoun and Tanimoto, 1991). 

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

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. 

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

Nitrate reductases are found in a wide range of eukaryotes and prokaryotes and have a crucial role in nitrogen assimilation and dissimilation (see Chapter 8.14). These enzymes catalyze the reaction shown in Equation (5) for the assimilatory nitrate reductases, this is followed by the reduction of nitrite to ammonia. Dissimilatory nitrate reductases [142 147] catalyze the reduction of nitrate to nitrite for respiration, to generate a transmembrane potential gradient.The assimilatory nitrate reductases have a molybdenum center similar to that of sulfite oxidase (see... 

Proposed reaction mechanism for the reduction of nitrate to nitrite by the dissimilatory nitrate reductase of Desulfovibrio desulfuricans ATCC 2774. ... 

Dissimilatory nitrate reductases (Pichinoty type A) in membrane fractions from bacteria have been shown capable of utilizing a variety of respiratory Intermediates and reduced pyridine nucleotides for nitrate reduction (Cole and Wimpeny, 1968 Knook et ai, 1973 Burke and Lascelles, 1975 Enoch and Lester, 1975). Reduction of nitrate by the membrane fractions, when respiratory substrates or pyridine nucleotides serve as reduc-tant, is generally inhibited by azide, cyanide and p-chloromercuribenzoate. Nitrate reduction, mediated by respiratory substrates, could be inhibited by n-heptylhydroxyquinoline-N oxide (HONO) or dicoumoral (Ruiz-Herrera and DeMoss, 1%9 Knook et al., 1973 Burke and Lascelles, 1975). However, in Micrococcus denitrificans (Lam and Nicholas, 1969) and in Bacillus stearothermophilus (Downey, 1%6) nitrate reduction is not inhibited by... 

Nitrate reductase enzymes which catalyse the reduction of nitrate to nitrite. All N.r. studied so far contain iron and molybdenum. In the sequence of electron transfer, molybdenum appears to be the ultimate acceptor, which then transfers electrons to nitrate during this process the molybdenum alternates between Mo(V) and Mo(VI). Dissimilatory N.r. from E. coii (also called respiratory N.r.) (EC 1.7.99.4) is a transmembrane protein, containing Mo, inorganic sulfur and nonheme iron, approximate M,... 

DiChristina TJ (1992) Effects of nitrate and nitrite on dissimilatory iron reduction by Shewanella putrefaciens 200. J Bacteriol 174 1891-1896. 

At this site in the eastern tropical North Pacific, denitrification is responsible fiar the midwater loss of nitrate and production of nitrite. The size of the secondary nitrite maximum is dependent on the relative rates of its production from NO3 and its loss via dissimilatory reduction to N2. The amount of nitrate lost to denitrification is shown as the difference between the measured nitrate and the calculated nitrate. The latter was estimated by multiplying the observed phosphate concentrations by the average nitrate-to-phosphate ratio in the three deepest samples (11.9 1.6pmolN/L). Note that the zone of denitrification is restricted to mid-depths, i.e., the depths of the OMZ at this site. 

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