Anaerobic respiration denitrification

Seventeen genera of facultative anaerobic bacteria (e.g., Pseudomonas and Bacillus) can perform denitrification under anaerobic or low-oxygen conditions, where they use NO3- as an electron acceptor during anaerobic respiration (Jaffe, 2000). In fact, in many estuaries, denitrification is limited by the availability of NC>3 (Koike and Sprensen, 1988 Cornwell et al., 1999). Sources of NC>3 and NC>2 for denitrification are from diffusive inputs from the overlying water column and nitrification in the sediments (Jenkins and Kemp, 1984). The activity of other bacterial processes under anoxic conditions has been shown to affect the activity of denitrifying bacteria. For example, SO42- reduction occurs in anoxic sediments whereby SC>42 is reduced to sulfide (Morse et al., 1992)—more... 

These reactions are the terminal electron-transfer reactions during anaerobic respiration, the enzymes being part of a redox loop generating a proton-motive force capable of driving ATP synthesis. Periplasmic nitrate reductase (Nap) participates in cellular redox processes, aerobic denitrification, and nitrate scavenging. ... 

The term anaerobic respiration describes oxidation of organic matter by inorganic electron acceptor other than O. Two of the most important inorganic electron acceptors are nitrate and sulfate and their reduction may be accompanied by calcium carbonate formation. In nitrate reduction (denitrification) by heterotrophs, either N2 or N2O are formed and the overall reduction-calcification processes are expressed by eqns (7) and (8) where C represents organic carbon ... 

Many organisms use oxidized nitrogen as a final electron acceptor in anaerobic respiration [33, 34]. This denitrification process can be described as a series of successive reducing steps NOJ NO2 NO N2O N2. 

The best example for facultative anaerobic autotrophic respiration is represented by Thiobacillus denitrificans, as shown in the denitrification reaction g ... 

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. 

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. 

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. 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

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. 

Anaerobic organisms often have the capacity to reduce two or more terminal electron acceptors. In many cases, these alternative reactions do not support growth, as with the fermenting bacteria that reduce Fe(ni) (Lovley, 2000b). In other cases, the ability to use multiple electron acceptors is presumably an adaptation for remaining active in an environment where the supply of specific electron acceptors is variable. For example, denitrification permits normally aerobic bacteria to respire in the absence of O2, albeit at a slower rate. 

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. 

Figure 28. Hypothetical anaerobic nitrogen cycle based on the following thermodynamically permissible reactions (1) ammonium oxidation to dinitrogen by carbon dioxide,. sulfate or ferric iron (no evidence at present, possibly kinetically limited) (2) dinitrogen fixation by various organic and inorganic reductants (known) (3) ammonium oxidation by nitrite or nitrate producing dinitrogen (known) (4) denitrification (known) (5) nitrite or nitrate respiration (known) (6) ferric iron oxidation of ammonium to nitrite or nitrate (no evidence at present) (7) nitrate assimilation (known) (8) ammonium assimilation and di.s,similation (known) (Fenchel etai, 1998).

Many bacterial genera contain denitrifying species Achromobacter, Alcaligenes (Alcaligenes odorans denitrifies nitrite). Bacillus, Chromobacterium, Coryne-bacterium, Halobacterium, Hyphomicrobium, Morax-ella, Paracoccus, Pseudomonas, Spirillum, Thiobacil-lus and Xanthomonas. In some species of Pseudomonas and Corynebacterium, N O is the final denitrification product. All these bacteria are aerobes that are able to respire (denitrify) nitrate under anaerobic conditions. The only true anaerobe able to carry out denitrification is Propionibacterium. There is no evidence for other intermediates in the above denitrification pathway, but in the formation of nitrous oxide (NjO) an NN bond must be formed, and there may exist transient enzyme-bound intermediates that have not yet been identified. The enzymology of denitrification from nitrite is poorly understood. It seems likely that each stage is linked to electron transport via a cytochrome system, but sites of ATP synthesis have not been unequivocally located. 

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