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Denitrifying bacteria are a diverse group of bacteria that encompass many different phyla. This group of bacteria, together with denitrifying fungi and archaea, is capable of performing denitrification as part of the nitrogen cycle. They metabolise nitrogenous compounds using various enzymes, turning nitrogen oxides back to nitrogen gas or nitrous oxide.

Diversity of denitrifying bacteria
There is a great diversity in bacteria capable of performing denitrification. Members of this group encompass most bacterial phyla and therefore posses a wide variety of physiological traits. Denitrifying bacteria have been identified in over 50 genera with over 125 different species and are estimated to represent 10-15% of bacteria population in water, soil and sediment. Denitrifying include for example several species of Pseudomonas, Alkaligenes, Bacillus and others. The majority of denitrifying bacteria are facultative aerobic heterotrophs that switch from aerobic respiration to denitrification when oxygen as an available terminal electron acceptor (TEA) runs out. This forces the organism to use nitrate to be used as a TEA. For the diversity of denitrifying bacteria is so large, this group can thrive in a wide range of habitats including some extreme environments such as environments that are high in saline and high in temperature.

Denitrification mechanism
Denitrifying bacteria use denitrification to generate ATP.

The most common denitrification process is outlined below, with the nitrogen oxides being converted back to gaseous nitrogen:

2 NO3− + 10 e- + 12 H+ → N2 + 6 H2O

The result is one molecule of nitrogen and six molecules of water. Denitrifying bacteria are a part of the N cycle, and consists of sending the N back into the atmosphere. The reaction above is the overall half reaction of the process of denitrification. The reaction can be further divided into different half reactions each requiring a specific enzyme. The transformation from nitrate to nitrite is performed by nitrate reductase (Nar)

NO3- + 2 H+ + 2 e- → NO2- + H2O

Nitrite reductase (Nir) then converts nitrite into nitric oxide

2 NO2- + 4 H+ + 2 e- → 2 NO + 2 H2O Nitric oxide reductase (Nor) then converts nitric oxide into nitrous oxide

2 NO + 2 H+ + 2 e- → N2O + H2O

Nitrous oxide reductase (Nos) terminates the reaction by converting nitrous oxide into dinitrogen N2O + 2 H+ + 2 e- → N2 + H2O

It is important to note that any of the products produced at any step can be exchanged with the soil environment.

Denitrifying bacteria and the environment
The process of denitrification can lower the fertility of soil as nitrogen, a growth-limiting factor, is removed from the soil and lost to the atmosphere. This loss of nitrogen to the atmosphere can eventually be regained via introduced nutrients, as part of the nitrogen cycle. Some nitrogen may also be fixated by species of nitrifying bacteria and the cyanobacteria. Another important environmental issue concerning denitrification is the fact that the process tends to produce large amounts of by-products. Examples of by-products are nitric oxide (NO) and nitrous oxide (N2O). NO is an ozone depleting species and NO2 is a potent greenhouse gas which can contribute to global warming.