Nitrate Reduction and Denitrification

A heterogeneous group of microorganisms, including many bacterial, fungal, and algal species, are capable of assimilatory nitrate reduction, a process that reduces nitrate and nitrite to ammonia, which can be subsequently incorporated into amino acids. 

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  

Primary denitrifying genera in soils are Pseudomonas and Alkaligens. Simultaneous with denitrification is organic matter oxidation. The utilization of glucose through nitrate reduction by Pseudomonas denitrificans can be written as 

Denitrification typically occurs under strictly anaerobic conditions or under low-oxygen partial pressures. 

Example 7.4 Determine the oxidation number of nitrogen in each species of the sequence 7.7. 

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Koike, I., and Sprensen, J. (1988) Nitrate reduction and denitrification in marine sediments. In Nitrogen Cycling in Coastal Marine Environments. SCOPE 33 (Blackburn, T.H., and Sprensen, J., eds.), pp. 251-274, John Wiley, New York. 

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.

Jprgensen B. B. and Sprensen J. (1988) Two annual maxima of nitrate reduction and denitrification in esmarine sediment (Norsminde Fjord, Denmark). Mar. Ecol. Prog. Ser. 48, 147-154. 

Explain how the CO2 content and pH of natural waters is affected by processes and reactions including the dissolution of C02(g), photosynthesis and respiration, aerobic decay, anaerobic decay (fermentation), nitrate reduction, and denitrification and sulfate reduction. 

Nishio, T., Koike, I., and Hattori, A. (1982) Denitrification, nitrate reduction, and oxygen consumption in coastal and estuarine sediments. Appl. Environ. Microbiol. 43, 648-653. 

Inorganic nitrogen nitrate, nitrite, and ammonium ions These ions are relevant parameters in environmental analysis because they are involved in important biological processes such as nitrate reduction, nitrite reduction and denitrification. Nitrate and nitrite reductases play a key... 

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. 

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).

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. 

Autotrophic activity. Because of the low C N ratio and its declining value as carbonaceous residues are degraded there is substantial ammonification. With all mean treatment times greater than the doubling time of Nitrobacter sp. nitrification will occur provided that oxygen is not limiting. Smith and Evans (19) found that with DO levels above 15% of saturation, nitrification continued until the culture was limited by a fall in pH level. Up to 40% of the slurry ammonia was oxidised. The autotrophic activity never achieved steady state and cycled between periods of activity when the pH value was above about 5.5 and periods of inactivity when the pH value fell below 5.5. Complete nitrification of all ammonia only occurred if the pH value was controlled at about 7 by the addition of alkali. When the DO level was held within the range of 1 to 15% of saturation a system of simultaneous nitrification and denitrification was established. The reduction of nitrate allowed the pH value to remain above 6 and nitrification to continue. Thus more than 70% of the ammonia was oxidised. If the DO level was held below 0.1% of saturation, nitrification was inhibited (unpublished). 

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. 

Tiedje JM. 1988. Ecology of denitrification and dissimUatory nitrate reduction to ammonium. In Zehnder AJB, ed. Biology of Anaerobic Microorganisms. New York Wiley-Interscience, 179-244. 

Tiedje, ]. M. (1988). Ecology of denitrification and dissimilatory nitrate reduction to ammonia. In Biology of Anaerobic Microorganisms (A. J. B. Zehnder, ed.). pp. 179-244. Wiley, New York. 

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. 

Nitrate and Ammonium. The transformations of nitrogen species may occur under suitable microbial catalysis (5, 36). Nitrate reduction may result in formation of either elemental nitrogen or ammonium. Mass balances over a whole lake have indicated the importance of the denitrification process for the elimination of nitrogen from lakes (37). The conditions for the dis-similative ammonification of nitrate are poorly known (36). Ammonium is also released by the mineralization of biomass. 

An, S., and Gardner, W.S. (2002) Dissimilatory nitrate reduction to ammonium (DNRA) as a nitrogen link, versus denitrification as a sink in a shallow estuary (Laguna Madre/Baffin Bay, Texas). Mar. Ecol. Prog. Ser. 237, 41-50. 

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