Diazotrophs are bacteria that fix atmospheric nitrogen gas into a more usable form such as ammonia (Postgate, 1998).
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 nitrogenase systems. Two of the most studied systems are those of Klebsiella pneumoniae and Azotobacter vinlandii. These systems are used because of their genetic tractability and their fast growth (Dixon and Kahn 2004).
Diazotrophs are scattered across bacterial taxonomic groups (mostly in the Eubacteria but also a couple of Archaea). Even within a species that can fix nitrogen there may be strains that do not fix nitrogen (Postgate, 1998). 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.
Anaerobes—these are obligate anaerobes that cannot tolerate oxygen even if they are not fixing nitrogen. They live in habitats low in oxygen, such as soils and decaying vegetable matter. Clostridium is an example. Sulphate-reducing bacteria are important in ocean sediments (e.g. Desulfovibrio), and some Archean methanogens fix nitrogen in muds and animal intestines (Postgate 1998).
Facultative anaerobes—these species can grow either with or without oxygen, but they only fix nitrogen anaerobically. Often, they respire oxygen as rapidly as it is supplied, keeping the amount of free oxygen low. Examples include Klebsiella pneumoniae, Bacillus polymyxa, Bacillus macerans, and Escherichia intermedia (Postgate, 1998).
Aerobes—these species require oxygen to grow, yet their nitrogenase is still debilitated if exposed to oxygen. Azotobacter vinelandii is the most studied of these organisms. It uses very high respiration rates, and protective compounds, to prevent oxygen damage. Many other species also reduce the oxygen levels in this way, but with lower respiration rates and lower oxygen tolerance (Postgate 1998).
Phototrophs—photosynthetic bacteria generate oxygen as a by-product of photosynthesis, yet some are able to fix nitrogen as well. These are colonial bacteria that have specialized cells (heterocysts) that lack the oxygen generating steps of photosynthesis. Examples are Anabaena cylindrica and Nostoc commune. Other cyanobacteria lack heterocysts and can fix nitrogen only in low light and oxygen levels (e.g. Plectonema) (Postgate 1998).
Rhizobia -- these are the species that associate with legumes, plants of the Fabaceae family. Oxygen is bound to leghemoglobin in the root nodules that house the bacterial symbionts, and supplied at a rate that will not harm the nitrogenase (Postgate 1998).
Frankias -- much less is known about these 'actinorhizal' nitrogen fixers. The bacteria also infect the roots and form nodule-like structures. Frankia forms heterocyst-like structures in these nodules where N-fixation occurs (Vessey et al., 2005). Frankias also produce hemoglobins (Beckwith et al., 2002), but their role is less well established than for rhizobia (vessey et al., 2005). Although at first it appeared that they infect sets of unrelated plants (alders, Australian pines, California lilac, bog myrtle, bitterbrush, Dryas), revisions to the phylogeny of angiosperms show a close relatedness of these species and the legumes (Soltis et al., 1995; Vessey et al. 2005).
Cyanobacteria -- there are also symbiotic cyanobacteria. Some associate with fungi as lichens, with liverworts, with a fern, and with a cycad (Postgate, 1998). These do not form nodules (indeed most of the plants do not have roots). Heterocysts exclude the oxygen, as discussed above. The fern association is important agriculturally: the water fern Azolla harbouring Anabaena is an important green manure for rice culture (Postgate, 1998).
Association with animals -- although diazotrophs have been found in many animal guts, there is usually sufficient ammonia present to suppress nitrogen fixation (Postgate 1998). Termites on a low nitrogen diet allow for some fixation, but the contribution to the termite's nitrogen supply is negligible. Shipworms may be the only species that derive significant benefit from their gut symbionts (Postgate 1998).
In terms of generating nitrogen available to all organisms, the symbiotic associations greatly exceed the free-living species with the exception of cyanobacteria (Postgate, 1998).
Beckwith, J, Tjepkema, J D, Cashon, R E, Schwintzer, C R, Tisa, L S (2002). "Hemoglobin in five genetically diverse Frankia strains". Can J Microbiol48: 1048-1055.
Dixon R and Kahn D (2004). "Genetic regulation of biological nitrogen fixation". Nat Rev Microbiol2 (8): 621-31.
Soltis DE, Soltis PS, Morgan DR, Swensen SM, Mullin BC, Dowd JM, Martin PG (1995). "Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms". Proc Natl Acad Sci USA92: 2647 – 2651.
Vessey JK, Pawlowski, K and Bergman B (2005). "Root-based N2-fixing symbioses: Legumes, actinorhizal plants, Parasponia sp and cycads". Plant and soil274 (1-2): 51-78.