Nitrite nitrogen cycle

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

In denitrification, part of the biological nitrogen cycle, nitrate in the soil is converted via four enzymatic reactions stepwise to nitrite, nitric oxide and nitrous oxide to finally yield gaseous nitrogen. 

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

Studies on nitrogen cycle enzymology in microorganisms that use ammonia as a major source of energy for growth suggest that HA serves as an intermediate in its oxidation to nitrite , as depicted in equations 1 and 2 ... 

The involvement of HA during bacterial conversion of nitrate to NH3 (known also as the nitrate ammonification phase of the nitrogen cycle) has been studied at the molecular level as part of an effort to delineate the mechanism of conversion of nitrite to NH3 by a group of multiheme cytochromes of bacterial origin. The overall reduction reaction is depicted in equation 3 for cytochrome c-nitrite reductase " ... 

The study of the biosynthesis of inorganic nitrogen oxides has a long and distinguished history, principally in the field of microbial and plant nitrogen metabolism (Zumft, 1993). Relatively recently, it has been found that NO, in addition to other species such as nitrite, nitrate, and NjO, is a true intermediate in the biological nitrogen cycle, as described in detail in Chapter 9. 

Finally we note that enzymatic reduction of N02 and N03 is important in the natural nitrogen cycle. Copper-containing reductases are known and, as models, copper(I) nitrite complexes with triazacyclononane ligands have been shown to evolve NO.54... 

Remember that in the natural nitrogen cycle (see Section 7.4.5), bacteria convert nutrient forms of nitrogen (i.e., ammonia and nitrite) back to nonnutritive N2 in the atmosphere. This process is called denitrification. For the biological removal of nitrogen, the activated sludge system is partitioned into zones, and the environment in each zone is... 

Treusch, A. H., Leininger, S., Kletzin, A., Schuster, S. C., iGenk, H. P., and Schleper, C. (2005). Novel genes for nitrite reductase and Amo-related proteins indicate a role of uncultivated meso-philic crenarchaeota in nitrogen cycling. Environmental Microbiology 7, 1985—1995. 

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

Figure 6 Microbial transformations of the nitrogen cycle. Pathways depicted are 1—N2 fixation 2—DIN assimilation 3—ammonium regeneration 4—nitrification 5—nitrate/nitrite reduction and 6—denitrification.

Figure 12 A diagram of the nitrogen cycle with catalyzing enzymes and metal requirements of each step. NIT, nitrogenase AMO, ammonium mono-oxygenase HAO, hydroxylamine oxidoreductase NAR, membrane-bound respiratory nitrate reductase NAP, periplasmic respiratory nitrate reductase NR, assimila-tory nitrate reductase NIR, respiratory nitrite reductase NiR, assimilatory nitrite reductase NOR, nitric oxide reductase N2OR, nitrous oxide reductase.

Nitrification is an aerobic microbial process by which specialized bacteria oxidize ammonium to nitrite and then to nitrate. Nitrification is a very important part of the nitrogen cycle, because for most plants nitrate is... 

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