Ammonia nitrate reduction, - respiration

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

The nitrite formed is either excreted directly or reduced by non-ATP-yielding reactions to ammonia. The enzyme machinery for both processes, nitrate/nitrite respiration and denitrification, is formed only under anaerobic conditions or conditions of low oxygen tension. In fact, the activities of the enzymes involved in dissimila-tory nitrate reduction are strongly inhibited by oxygen. Thus, denitrification and nitrate/nitrite respiration take place only when oxygen is absent or available in insufficient amounts. 

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... 

N2O Production by Nondenitrifiers During the study of nitrate reduction to ammonia by nitrate-respiring organisms, we noted N2O but not N2 production by all of the cultures listed above except Clostridiurriy which produced no gas. We saw no evidence of further consumption of the N2O. Using as substrate, we evaluated the ratio of... 

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. 

Figure 1. Electron free energy levels calculated for the approximate pH of the oxic-anoxic interface of the Black Sea (pH 7.75). Dissolved species other than H are assumed to have unit activity. The strongest oxidants are at the top, and the strongest reductants are at the bottom. Such diagrams are a simple way to evaluate the feasibility of redox reactions. For example, ammonia and Mn2+ oxidation by nitrate may be feasible, but the actual free energy available will depend on the in situ concentrations at the site of reaction. All such reactions are, most likely, mediated by bacteria. The vertical separation of the different oxidants from organic matter (CH20) is proportional to the energy available from the different respiration reactions (1).

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... 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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