What is Nitrogen fixation?
'''Nitrogen fixation''' is how [[nitrogen]] in the air (N<sub>2</sub>) is changed (converted) into ammonia ({{chem|NH|3}}) or other nitrogenous compounds in the [[soil]]. Nitrogen fixation is essential to life because nitrogen compounds are needed for making all nitrogen-containing [[organic compounds]], such as [[amino acids]], [[protein]]s, and [[nucleic acid]]s. Atmospheric nitrogen is a relatively unreactive molecule. It is useless to all except a few [[bacteria]] and [[archaea]]. Biological nitrogen fixation converts {{chem|N|2}} into [[ammonia]], which is used (metabolized) by most organisms. Nitrogen is fixed in biological and non-biological ways: == Biological == ===Microorganisms that fix nitrogen (diazotrophs)=== * [[Cyanobacteria]], ''e.g.'' the highly significant ''[[Trichodesmium]]'' * [[Green sulfur bacteria]] * ''[[Azotobacteraceae]]'' * ''[[Rhizobia]]'' * ''[[Frankia]]'' [[Gram-positive]] soil bacteria cause nitrogen-fixing nodules in [[actinorhizal plant]]s. [[Cyanobacteria]] are in most environments on Earth. They play key roles in the carbon and [[nitrogen cycle]] of the [[biosphere]]. Cyanobacteria use many sources of combined nitrogen, like [[nitrate]], [[nitrite]], [[ammonium]], [[urea]], or some [[amino acids]]. Several cyanobacteria are also diazotrophs which can fix nitrogen from the air. This is an ability which may have been present in their last common ancestor in the [[Archaea]]n.<ref>Latysheva N. ''et al'' 2012. The evolution of nitrogen fixation in cyanobacteria. ''Bioinformatics''. '''28'''(5) 603–606; {{doi|10.1093/bioinformatics/bts008}}</ref> Cyanobacteria in [[coral reef]]s can fix twice the amount of nitrogen than on land—around 1.8&nbsp;kg of nitrogen is fixed per hectare per day. The colonial marine cyanobacterium ''Trichodesmium'' may fix nitrogen on such a scale that it accounts for almost half of the nitrogen-fixation in marine systems on a global scale.<ref>{{cite journal|author=Bergman B. 2012 |title=''Trichodesmium'' – a widespread marine cyanobacterium with u