Biological nitrogen fixation

Biological nitrogen fixation is the microbial conversion of atmospheric nitrogen gas into ammonia, a form that can enter biological nitrogen cycles. It is carried out by nitrogenase-containing microorganisms, either independently or in association with plants.

Nitrogenase: A metal-containing enzyme complex that reduces atmospheric nitrogen to ammonia. It catalyzes the defining chemical reaction of biological nitrogen fixation.
Nitrogen: A chemical element essential to proteins, nucleic acids, and many other biological molecules. Its atmospheric form is abundant but unavailable to most organisms without fixation.
Rhizobium: A group of soil bacteria that form nitrogen-fixing symbioses with legume plants. These bacteria supply fixed nitrogen to crops through root-nodule partnerships.
Nitrogen limitation: A condition in which nitrogen availability constrains organismal growth or ecosystem productivity. By adding usable nitrogen, fixation can relieve a common limit on biological production.
Nitrogenase iron protein: The ATP-powered component of conventional nitrogenase that transfers electrons to its catalytic partner. Its repeated ATP-driven electron transfers supply nitrogenase with reducing power.
Dinitrogen: A molecule composed of two nitrogen atoms, making up most of Earth's atmosphere. Its strong triple bond makes atmospheric nitrogen difficult for organisms to use.
Legume: A plant in the family Fabaceae, many of which form nitrogen-fixing root-nodule symbioses. Legumes host microbial fixation and can enrich nitrogen in agricultural systems.
Reactive nitrogen: Nitrogen compounds other than atmospheric dinitrogen that participate in biological and chemical processes. Fixation moves nitrogen from inert atmospheric gas into the reactive nitrogen pool.
Nitrogenase molybdenum-iron protein: The catalytic component of conventional nitrogenase that binds and reduces nitrogen. Its active site carries out the conversion of nitrogen into ammonia.
Nitrogen cycle: The movement of nitrogen among the atmosphere, organisms, soils, and waters through chemical and biological processes. Fixation is the main natural route that brings atmospheric nitrogen into many ecosystems.

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