Nitrogen fixation

Nitrogen fixation is a process by which nitrogen in the air is converted into ammonia (NH3) or related nitrogenous compounds. Atmospheric nitrogen, is molecular dinitrogen (N2), a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N2 into ammonia, which is metabolized by most organisms. Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all chemical compounds that contain nitrogen, which includes explosives, most pharmaceuticals, and dyes. Nitrogen fixation is carried out naturally in the soil by a wide range of microorganisms termed diazotrophs that include bacteria such as Azotobacter, and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with some plant groups, especially legumes. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation also occurs between some termites and fungi. It also occurs naturally in the air by means of NOx production by lightning. All biological nitrogen fixation is effected by enzymes called nitrogenases. These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium.


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