Nitrogen Cycle

Nitrogen Cycle

Microorganisms, in the period of their growth and metabolism, interact with one another within the cycling of nutrients, including carbon, nitrogen, phosphorus, sulfur, iron, and manganese.

This nutrient cycling, called biogeochemical cycling when applied to the environment, involves both biological and chemical processes All of the biogeochemical cycles are linked and the metabolism-related transformations of these nutrients make life on Earth possible.

Nitrogen Cycle fix the the inorganic element (N2) to its organic form (NH4+amino acids). Nitrogen fixation is a uniquely procaryotic process apart from a limited amount of nitrogen fixation that occurs during lightning strikes, all organic nitrogen is of procaryotic origin. Nitrogen fixation can be carried out under oxic and anoxic condition.

nitrogen cycle
Flows that occur predominantly under oxic conditions are noted with open arrows. Anaerobic processes are noted with solid bold arrows. Processes occurring under both oxic and anoxic conditions are marked with cross-barred arrows. The anammox reaction of NO2 and NH4+ to yield N2 is shown. Important genera contributing to the nitrogen cycle are given as examples.


Microbes such as Azotobacter and the cyanobacterium Trichodesmium fix nitrogen aerobically, while free-living anaerobes such as members of the genus Clostridium fix nitrogen anaerobically.

The best-studied nitrogen-fixing microbes are the bacterial symbionts of leguminous plants, including Rhizobium, its α-proteobacterial relatives, and some recently discovered β-proteobacteria (e.g., Burkholderia and Ralstonia spp.).

The actinomycete Frankia fixes nitrogen while colonizing many types of woody shrubs, and the heterocystous cyanobacterium Anabaenea fixes nitrogen when in association with the water fern

The product of N2 fixation is ammonia (NH3); it is instantly incorporated into organic matter as an amine. The addition of eight electrons per N atom requires a great deal of energy and reducing power. The nitrogenase enzyme is thus very sensitive to O2 and must be protected from oxidizing conditions.

Aerobic and microaerophilic nitrogen-fixing bacteria employ a number of strategies to protect their nitrogenase enzymes. For example, heterocystous cyanobacteria physically separate nitrogen fixation from oxygenic photosynthesis by confining the process to special cells called heterocysts, while other cyanobacteria fix nitrogen only at night when photosynthesis is impossible.

Ammonia made by N2 fixation is instant mix with organic matter as amines. This type of amine N-atoms are present into proteins, nucleic acids, and various biomolecules.
The N cycle continues with the degradation of these molecules into ammonium (NH4+) within mixed assemblages of microbes.

nitrogen cycle

One important fate of this ammonium is its conversion to nitrate (NO3), a process called nitrification. This is a two step process whereby ammonium ion (NH4+) is first oxidized to nitrite (NO2), which is then oxidized to nitrate.

Bacteria of the genera Nitrosomonas and Nitrosococcus, for example, play important roles in the first step, and Nitrobacter and related chemolithoautotrophic bacteria carry out the second step.

In addition, Nitrosomonas eutropha has been found to oxidize ammonium ion anaerobically to nitrite and nitric oxide (NO) using nitrogen dioxide (NO2) as an acceptor in a denitrification related reaction.

The production of nitrate is important because it can be reduced and incorporated into organic nitrogen; this process is known as assimilatory nitrate reduction. The use of nitrate as a source of organic nitrogen is an example of assimilatory reduction.

Because assimilatory reduction of nitrate to ammonium is energetically expensive, nitrate sometimes accumulates as a transient intermediate. Alternatively, for some microbes nitrate provides as a terminal electron acceptor through anaerobic respiration this is a form of dissimilatory reduction.

In this case, nitrate is completely exit from the ecosystem and again returned to the atmosphere as dinitrogen gas (N2) through a series of reactions that are known as denitrification.

This dissimilatory process, in which nitrate is utilised as an electron acceptor in anaerobic respiration (in the absebse of oxygen), mainly involved heterotrophs like Pseudomonas denitrificans.

The major products of denitrification include nitrogen gas (N2) and nitrous oxide (N2O), although nitrite (NO2) also can accumulate. Nitrite is of environmental concern because it can contribute to the formation of carcinogenic nitrosamines.

Finally, nitrate can be transformed to ammonia in dissimilatory reduction by a various kind of bacteria, includes Geobacter metallireducens, Desulfovibrio spp. and Clostridium spp.

A recently identified form of nitrogen conversion is called the anammox reaction (anoxic ammonium oxidation).

In effect, the anammox reaction is a shortcut to N2, begin directly from ammonium and nitrite without having to cycle first through nitrate. Although this reaction was known to be energetically
possible, microbes capable of performing the anammox reaction were only recently documented.

The discovery that marine bacteria perform the anammox reaction in the anoxic waters just below oxygenated regions in the open ocean solved a longstanding mystery.

For many years microbiologists wondered where the “missing” NH4+ could be—mass calculations did not agree with experimentally derived nitrogen measurements. The discovery that planctomycete bacteria oxidize measurable amounts of NH4+ to N2, thereby removing it from the marine ecosystem, has necessitated a reevaluation of nitrogen cycling in the open ocean.

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Nitrogen Cycle

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