Assimilatory nitrogen oxide reduction

Nitrogen uptake that results in the formation of new biomolecules is termed an assimilation process, such as assimilatory nitrogen reduction. The processes that result in the release of DIN into seawater are referred to as dissimilations, such as dissimi-latory nitrogen reduction. An example of the latter is denitrification, in which nitrate and nitrite obtained from seawater serve as electron acceptors to enable the oxidation of organic matter. This causes the nitrate and nitrite to be transformed into reduced species, such as N2O and N2, which are released back into seawater. 

The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

This process is commonly referred to as assimilatory nitrogen (nitrate or nitrite) reduction. The electrons for these reductions are supplied by half-cell oxidations involving NADPH/NADP" and NADH/NAD" (Table 7.11). All of these reactions and membrane transport processes are mediated by enzymes that are specific to the DIN species. Considerable variation exists among the phytoplankton species in their ability to produce the necessary enzymes. Since marine phytoplankton are often nitrogen limited, the quantity and type of DIN available in the water column can greatly influence overall phytoplankton abundance and species diversity. 

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

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 12 Major reduction-oxidation reactions involving nitrogen. The reactions are numbered as follows (1) mineralization, (2) ammonium assimilation, (3) nitrification, (4) assimilatory or dissimilatory nitrate reduction, (5) ammonium oxidation, (6) nitrite oxidation, (7) assimilatory or dissimilatory nitrate reduction, (8) assimilatory or dissimilatory nitrite reduction, (9) denitrification, (10) chemodenitrification, (11) anaerobic ammonium oxidation, and (12) dinitrogen fixation (after Capone, 1991) (reproduced by permission of ASM Press from Microbial Production and Consumption of Greenhouse Gases Methane, Nitrogen Oxides, and Halomethanes, 1991).

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. 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Oxidized forms of nitrogen such as nitrate, need to first be reduced to ammonium before their incorporation into biomass. Nitrate is first reduced by the enzyme assimdatory nitrate reductase to nitrite. Assimilatory nitrite reduction subsequendy reduces the nitrite to ammonium (D Elia and Webb, 1977) (Fig. 21.IE) (see Chapter 7, MuUioUand and Lomas, this volume). 

The near absence of 02 results in vigorous denitrification [bacterial conversion of N03 to molecular nitrogen (N2)] within a layer that is several hundred metres thick and spreads over an area measuring around 1.4x 106km2 (Naqvi, 1994). This zone can be easily identified by the accumulation of nitrite (N02 ), the first intermediate of the reduction sequence (N03 N02 — N0 N20— N2) (Figs 6.14b and 6.15a). Note that N02 is also produced through assimila-tory reduction of N03 by phytoplankton and the oxidation of NH4+ (nitrification), but both these processes occur in oxic waters close to the surface (Codispoti Christensen, 1985). While the primary N02 maximum resulting from nitrification/assimilatory N03- reduction is ubiquitously found near the base... 

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