Denitrification anaerobic conditions

Garrido, J.M., Mendez, R., and Lema, J.M., Simultaneous urea hydrolysis, formaldehyde removal and denitrification in a multifed upflow filter under anoxic and anaerobic conditions, Water Res., 35, 691-698, 2001. 

Carbon acts as the electron donor for denitrification. The availability of carbon often limits denitrification in anaerobic microsites in non-submerged soils. As a result, the reaction does not go to completion and the intermediaries NO2 and N2O accumulate. Completion of the reaction may also be hindered by low pH. But under uniformly anaerobic conditions NO3 as electron acceptor is more likely to be limiting than carbon as electron donor because NO3 is not regenerated. 

In a soil-water system under anaerobic conditions, no significant degradation of naphthalene was observed after 45 d. Under denitrification conditions, naphthalene (water concentration 700 pg/L) degraded to nondetectable levels in 45 d. In both studies, the acclimation period was approximately 2 d (Mihelcic and Luthy, 1988). 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

N2 under anaerobic conditions, a process called denitrification (Fig. 22-1). These soil bacteria use N03 rather than 02 as the ultimate electron acceptor in a series of reactions that (like oxidative phosphorylation) generates a transmembrane proton gradient, which is used to synthesize ATP. 

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. 

Denitrification The reduction of nitrates to nitrites and finally to nitrous oxide or even to molecular nitrogen catalyzed by facultative aerobic soil bacteria working under anaerobic conditions. 

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. 

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

Heterotrophic side reaction stage—The initial stage of denitrification where heterotrophs consume the last amounts of dissolved oxygen before anaerobic conditions finally set in. 

Denitrification is the reduction of N03 N02 NO N2O —> N2 gas that is mediated by bacteria under anaerobic conditions, most generally in microbial mats and sediments. There are a number of methods to measure denitrification acetylene inhibition, isotope pairing, changes in N2 fluxes, and changes in the N2 to argon (Ar) ratio. Each of the techniques has their pros and cons and none is clearly superior under all conditions (see reviews by Cornwell et al, 1999 Chapter 6 by Devol, this volume). 

In the acetylene inhibition technique, acetylene is added to a water sample, which inhibits the reduction of N2O to N2 (Sorensen, 1978). The accumulation of N2O is then measured using gas chromatography and an electron capture detector and the denitrification rate is taken to be equal to the total N2O flux. One potential problem is incomplete inhibition of N2O reduction to N2, particularly in the presence of hydrogen sulfide, a compound commonly found under anaerobic conditions. Another potential problem with the technique is that acetylene also inhibits nitrification, a process that often supplies the NOs and N02 substrates for denitrification. To inhibit nitrification is to inhibit denitrification if it is at aU substrate limited (Hynes and Knowles, 1978). 

Denitrification can be limited by carbon availability when O2 is absent and NO3 is abundant. Additions of glucose stimulated denitrification in 11 of 13 agricultural soils that were presumably fertilized (Drury etaL, 1991). Similar observations have been made in water columns (Brettar and Rheinheimer, 1992), marine sediments (Slater and Capone, 1987), river sediments (Bradley et aL, 1995), aquifers (Smith and Duff, 1988 Obenhuber and Lowrance, 1991), wastewater treatment wetlands (Ingersoll and Baker, 1998), and forested wetlands (DeLaune et aL, 1996). Tiedje (1988) proposed that the major influence of carbon on in situ denitrification is to promote anaerobic conditions. 

Because nitrification occurs only under aerobic (or microaerobic) conditions and denitrification under anaerobic conditions, the two processes are spatially separated. However, if the sites where these processes occur are sufficiently close together, NO transport and consumption are very rapid and the overall process is considered to be coupled nitrification-denitrification. On the basis of a literature review, Seitzinger (1988) concluded that nitrification is generally the major source of NOj" for denitrification in river, lake, and coastal sediments. The same is likely to be true of non agricultural soils that are largely dependent on mineralization and atmospheric deposition for fixed nitrogen. 

Classical coupled nitrification-denitrification requires NH, organic carbon, aerobic conditions, and anaerobic conditions. It involves three distinct populations of microorganisms, some of which are heterotrophic and others autotrophic. As a result, regulation of the process is rather complex and the relative proportions of NOj", NO, N2O, and N2 as end products varies widely with environmental and ecological conditions. 

The anaerobic and aerobic nitrifier denitrification pathways differ in that NO is an end product under anaerobic conditions rather than an intermediate compound. In addition, nitrogen dioxide-dependent NH3 oxidation by N. eutropha does not require ammonium monooxygenase (Schmidt et al., 2002), demonstrating that the two pathways are enzymatically different. In the absence of NH3, N. eutropha can use H2 or simple organic compounds as electron donors (Abeliovich and Vonhak, 1992 Bock et al., 1995). In contrast to the anammox process, which is strictly anaerobic, O2 does not inhibit N02-dependent NH3 oxidation and N2 production can occur even under aerobic conditions (Zart and Bock, 1998). However, Shrestha et al. (2002) observed N2 production... 

If soils with large nitrate concentrations become wet and anaerobic conditions develop, the rate of denitrification is greatly increased. Denitrification represents a loss of fixed nitrogen capital, and the emitted nitrous oxide (N2O) may contribute to an enhancement of Earth s greenhonse effect. 

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