Nitrogen fixing bacteria

Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.

A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are rhizobia and Frankia (in symbiosis) and Azospirillum. All diazotrophs contain iron-molybdenum or -vanadium nitrogenase systems. Two of the most studied systems are those of Klebsiella pneumoniae and Azotobacter vinelandii. These systems are used because of their genetic tractability and their fast growth.

Diazotrophs are scattered across bacterial taxonomic groups (mostly in the Bacteria but also a couple of Archaea). Even within a species that can fix nitrogen there may be strains that do not fix nitrogen. Fixation is shut off when other sources of nitrogen are available, and, for many species, when oxygen is at high partial pressure. Bacteria have different ways of dealing with the debilitating effects of oxygen on nitrogenases, listed below.

In terms of generating nitrogen available to all organisms, the symbiotic associations greatly exceed the free-living species with the exception of cyanobacteria.

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