Denitrification

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Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen (N₂) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favourable include: nitrate (NO₃^-), nitrite (NO₂^-), nitric oxide (NO), and nitrous oxide (N₂O). In terms of the general nitrogen cycle, denitrification performs the opposite function of nitrogen fixation, which converts gaseous nitrogen into a more oxidised and biologically available form. The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g.,Thiobacillus denitrificans). Denitrifiers are represented in all main proteolytic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.^[1]

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:

NO₃^- → NO₂^- → NO → N₂O → N₂ gas

Or expressed as a redox reaction:

2NO₃^- + 10e^- + 12H⁺ → N₂ + 6H₂O

Denitrification is the second step in the nitrification-denitrification process, the conventional way to remove nitrogen from sewage and municipal wastewater. It is also an instrumental process in wetlands and riparian zones for the removal of excess nitrate from groundwater with excess nitrate levels, commonly by extensive agricultural or residential fertiliser usage.

Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction.

Reduction under anoxic conditions can also occur through process called anaerobic ammonia oxidation (Anammox), this reaction is expressed as the following:

NH₄⁺ + NO₂^- → N₂ + 2H₂O

In some wastewater treatment plants, small amounts of methanol are added to the wastewater to provide a carbon source for the denitrification bacteria.