Nitrogen fixation

Nitrogen fixation is a chemical process by which the abundant but relatively inert molecular dinitrogen (N2) is converted into bioavailable nitrogen compounds such as ammonia (NH3) and nitrates (NO3). It occurs both biologically and abiologically, the latter either naturally through weather phenomena (e.g. lightning) or artificially by chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases, produced by microorganisms such as cyanobacteria and rhizobia. These enzyme complexes are encoded by the Nif genes (or Nif homologs) and contain iron, often with a second metal (usually molybdenum, but sometimes vanadium). Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses, and aquatic ferns such as Azolla. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NOx production by lightning. Nitrogen fixation is a key component of the nitrogen cycle and is essential to life on Earth, as all vital organic compounds such as DNA and proteins contain nitrogen. Industrial nitrogen fixation, most notably through the Haber process, underpins the manufacture of all nitrogenous industrial products, which include fertilizers, pharmaceuticals, textiles, dyes and explosives.