Diazotroph
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.
Free-living diazotrophs
*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, B. 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).
Symbiotic diazotrophs
*
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, bitter brush,
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 sequester 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).
http://azotobacter.org Azotobacter
http://www.rhizobia.co.nz Rhizobia
http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm Frankia & Actinorhizal Plants
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