Ammonium anaerobic oxidation with nitrate

In continental shelf sediments, up to 67% of the N2 formation was found to be due to anaerobic ammonium oxidation with nitrate (or possibly nitrite) and only 33% of the N2 production was due to denitrification (Thamdrup and Dalsgaard, 2002). At present this reaction is not reported in wetland soils. Anaerobic ammonium oxidation is regulated by... 

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.

Anaerobic Oxidation of Ammonium with Nitrate (Anammox)... 

Redox potential It is usually considered that no autotrophic nitrification will occur when the redox potential is below 250 mV. Under mildly aerobic conditions, the oxidation of ammonium to nitrate is carried out joiutly by autotrophic and heterotrophic bacteria, whereas under anaerobic conditions with the redox potential below -85 mV, only heterotrophic nitrification takes place. 

Another mechanism of fixed N loss that occurs in sediments and the water column is anaerobic ammonium oxidation, or anammox , in which nitrite (from nitrate reduction or ammonium oxidation) is used to oxidize ammonium to N2 (N02 + Nl-l4+ N2+ 2H2O). This process has unknown effects on isotope distributions in the ocean. The effects of anammox on N isotopes must depend on the organism-scale isotope effects, the sources of nitrite and ammonium substrates for the reaction, and the degree to which these substrates are consumed. For instance, if nitrate reduction by denitrifiers is the source of the nitrite, remineralization processes are the source of the ammonium, and both the nitrite and ammonium are completely consumed in the environment where anammox occurs, then the isotope discrimination would simplify to that of the nitrate reduction by denitrifiers averaged with any isotope discrimination during the remineralization that produces the needed ammonium. It should be noted that many water-column-derived... 

Major limiting factors controlling anaerobic ammonium oxidation reaction are the availability of nitrite and the competition for electron acceptors by heterotrophs. Soils and sediments with high organic matter content can create higher demand for electron acceptors (nitrite and nitrate). Under these conditions, anammox may not be able to keep up with denitrification when electron donor availability is very high. It is likely under available carbon-limiting conditions that some autotrophs... 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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