Nitrate reduction pathway

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. 

When soybean leaves and pine needles were exposed to ozone, there was an initial decrease in the levels of soluble sugars followed by a subsequent increase. Ozone exposure also caused a decrease in the activity of the glycolytic pathway and the decrease in the activity was reflected in a lowered rate of nitrate reduction. Amino acids and protein also accumulated in soybean leaves following exposure. Ozone increased the activities of enzymes involved in phenol metabolism (phenylalanine ammonia lyase and polyphenoloxidase). There was also an increase in the levels of total phenols. Leachates from fescue leaves exposed to ozone inhibited nodulation. 

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. 

Sprensen, J. (1987) Nitrate reduction in marine sediment pathways and interactions with iron and sulfur cycling. Geomicrobiol. J. 5, 401 -21. 

Denitrifying nitrate reducers such as Paracoccus denitrificans, Thiobacillus denitrificans, and various pseudomonads follow a more complete reduction pathway, converting nitrate through nitrite to nitric oxide, nitrous oxide and lastly to molecular nitrogen ... 

Because NTR links the reduced and oxidized sides of the N cycle, it can be considered a central process that provides substrate to microbes that employ nitrate or nitrite as oxidant (see Chapter 5 by Ward, this volume Fig. 19.1, arrow 4). Like NH4, the products of NTR, N02, and NOs , may experience one of several possible fates, including (1) flux from the sediment, (2) assimilation within the sediment or at the sediment—water interface, or (3) reduction by one of three possible dissimilatory pathways DNF, dissimilatory nitrate reduction to ammonium (DNRA), or ANAM (Fig. 19.1, arrows 5, 6, and 7 Fig. 19.2). Uptake of NO by... 

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. 

Laboratory work suggests, however, that Fe limitation probably does not directly inhibit N03 uptake by compromising nitrate reductase activity (Milligan and Harrison, 2000). Instead, Fe limitation may block the N03 reduction pathway further downstream, by preventing nitrite (N02 ) reduction to NH4+ due to insufficient supplies of photosyntheticaUy-produced reductant. This study showed that Fe-deficient diatom cells continue to take up and reduce N03 to nitrite. 

Dissimilatory nitrate reduction to ammonium is an anaerobic pathway that is insensitive to NH4 and yields energy. The first step of the process is termed nitrate respiration because it is coupled to electron transport phosphorylation that generates ATP ... 

Chemoautotrophic denitrification coupled to H2S, S°, or 8203 occurs in some bacteria of the genus Thiobacillus such as T. denitrificans (Hoor, 1981 Section 8.08.7.9). Nitrate reduction by such a pathway increased with FeS additions and followed Michaelis-Menten kinetics in a marine sediment (Garcia-Gil and Golterman, 1993). 

Bonin P. (1996) Anaerobic nitrate reduction to ammonium in two strains isolated from coastal marine sediment a dissimilatory pathway. FEMS Microbiol. Ecol. 19, 27-38. 

Two different pathways can be monitored for nitrates in terrestrial and aquatic ecosystems. The first is related to assimilatory nitrate reduction and the second to denitrification. 

Chemical reactions in aquatic media that are promoted and catalyzed by microbes occur through a number of pathways (Fig. 2). Reductants produced by photosynthetic light reactions drive the metabolic processes of autotrophic organisms, including carbon fixation and nitrate reduction ... 

Unknown intermediate steps exist between the nitrite (NOJ), N2, and NH3. forms. The fraction denitrified and the N2O/N2 ratio increase with increasing amounts of initial NOJ. Nitrate reduction is the main pathway of denitrification, N2 and N2O production, in soils. 

Nitrate reducers are one of the most widely studied microbial groups, largely because of their role in N cycling (see Chapter 8 for a detailed discussion on this process). Nitrate reduction in wetlands follows the following pathways ... 

FIGURE 8.5 Oxidation and reduction reactions of nitrogen in wetlands. Numbers 1-7 refer to pathways of nitrogen reactions. 1 = ammonification 2 = immobilization 3 = nitrification 4 = denitrification 5 = dissimilatory nitrate reduction to ammonia 6 = dinitrogen fixation and 7 = ammonia volatilization. 

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