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Denitrification
 
 
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Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
 (N2) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 oxidized forms of nitrogen in response to the oxidation of an electron donor
Electron donor

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
 such as organic matter. The preferred nitrogen electron acceptor
Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
s in order of most to least thermodynamically favourable include: nitrate
Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates....
 (NO3-), nitrite
Nitrite

The nitrite ion is NO2-. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a Salt or an ester of nitrous acid....
 (NO2-), nitric oxide
Nitric oxide

Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NitrogenOxygen. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry....
 (NO), and nitrous oxide
Nitrous oxide

Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....
 (N2O).






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Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
 (N2) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 oxidized forms of nitrogen in response to the oxidation of an electron donor
Electron donor

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
 such as organic matter. The preferred nitrogen electron acceptor
Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
s in order of most to least thermodynamically favourable include: nitrate
Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates....
 (NO3-), nitrite
Nitrite

The nitrite ion is NO2-. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a Salt or an ester of nitrous acid....
 (NO2-), nitric oxide
Nitric oxide

Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NitrogenOxygen. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry....
 (NO), and nitrous oxide
Nitrous oxide

Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....
 (N2O). In terms of the general nitrogen cycle
Nitrogen cycle

The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a cycle which includes Gas components....
, denitrification completes the cycle by returning N2 to the atmosphere. The process is performed primarily by heterotrophic bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
 (such as Paracoccus denitrificans
Paracoccus denitrificans

Paracoccus denitrificans, a Gram-negative, coccoid, denitrifying bacterium, formerly known as Micrococcus denitrificans, was first isolated in 1910 by Martinus Beijerinck ....
 and various pseudomonads
Pseudomonas

Pseudomonas is a genus of gamma proteobacteria, belonging to the larger family of pseudomonads.Recently, 16S ribosomal RNA sequence analysis has redefined the taxonomy of many bacterial species....
), although autotrophic denitrifiers have also been identified (e.g.,Thiobacillus denitrificans). Denitrifiers are represented in all main phylogenetic 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.

Denitrification takes place under special conditions in both terrestrial and marine ecosystem
Ecosystem

An ecosystem is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all of the non-living physical factors of the environment....
s. In general, it occurs where oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, a more energetically favourable electron acceptor, is depleted, and bacteria respire
Respiration (physiology)

In animal physiology, respiration is the transport of Oxygen from the outside air to the cells within Tissue s and the transport of carbon dioxide in the opposite direction....
 nitrate as a substitute terminal electron acceptor
Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
. 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 soil
Soil

Soil is the naturally occurring, unconsolidated or loose covering on the Earth's surface. Soil is composed of particles of broken rock that have been altered by chemical and environmental processes including weathering and erosion....
s and groundwater
Groundwater

Groundwater is water located beneath the ground surface in soil porosity spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water....
, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:
NO3-
Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates....
 ? NO2-
Nitrite

The nitrite ion is NO2-. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a Salt or an ester of nitrous acid....
 ? NO
Nitric oxide

Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NitrogenOxygen. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry....
 ? N2O
Nitrous oxide

Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....
 ? N2
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
 gas


The complete denitrification process can be expressed as a redox
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 reaction:
2NO3- + 10e- + 12H+ ? N2 + 6H2O


This reaction shows a fractionation in isotope
Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
 composition. Lighter isotopes of nitrogen are preferred in the reaction, leaving the heavier nitrogen isotopes in the residual matter. The process can cause delta-values of up to -40, where delta is a representation of the difference in isotopic composition. This can be used to identify denitrification processes in nature.

Denitrification is commonly used to remove nitrogen from sewage
Sewage

Sewage is the mainly liquid waste containing some solids produced by humans which typically consists of washing water, feces, urine, laundry waste and other material which goes down Plumbing fixture from households and industry....
 and municipal wastewater
Wastewater

Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations....
. It is also an instrumental process in wetlands and riparian zones for the removal of excess nitrate from groundwater resulting from excessive agricultural or residential fertiliser usage.

Direct reduction from nitrate to ammonium
Ammonium

The ammonium cation is a positively electric charge polyatomic ion of the chemical formula NH4+. It has a formula weight of 18.05 and is formed by protonation of ammonia ....
, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
. 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
Anammox

Anammox, an abbreviation for anaerobic ammonium oxidation, is a stage in the nitrogen cycle. In this biological process, nitrite and ammonium are converted directly into dinitrogen gas....
), this reaction is expressed as the following:
NH4+ + NO2- ? N2 + 2H2O


In some wastewater treatment plant
Wastewater treatment plant

Wastewater treatment plant also called wastewater treatment works can mean one of the following:*Sewage treatment ? treatment and disposal of human waste....
s, small amounts of methanol
Methanol

Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a chemical compound with chemical formula carbonhydrogen3oxygenhydrogen ....
 are added to the wastewater to provide a carbon source for the denitrification bacteria.

Literature

Atlas, R.M., Barthas, R. Microbial Ecology: Fundamentals and Applications. 3rd Ed. Benjamin-Cummings Publishing. ISBN: 0805306536

Zumft, W.G. (1997): Cell biology and molecular basis of denitrification. In: Microbiol. Mol. Biol. Rev. Bd. 61, Nr. 4, S. 533-616. PMID 9409151

See also

  • Anaerobic respiration
    Anaerobic respiration

    Anaerobic respiration is the process of generating energy through cellular respiration , without the use of oxygen....
  • Anammox
    Anammox

    Anammox, an abbreviation for anaerobic ammonium oxidation, is a stage in the nitrogen cycle. In this biological process, nitrite and ammonium are converted directly into dinitrogen gas....
  • Bioremediation
    Bioremediation

    Bioremediation can be defined as any process that uses microorganisms, fungi, phytoremediation or their enzymes to return the natural environment altered by contaminants to its original condition....
  • Nitrogen cycle
    Nitrogen cycle

    The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a cycle which includes Gas components....
  • Sewage treatment
    Sewage treatment

    Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic....