Diazotroph

Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.[1]

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.[2]

Types of diazotrophs

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.[1] 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

Symbiotic diazotrophs

Importance

In terms of generating nitrogen available to all organisms, the symbiotic associations greatly exceed the free-living species with the exception of cyanobacteria.[1]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK.
  2. Dixon R and Kahn D (2004). "Genetic regulation of biological nitrogen fixation". Nat Rev Microbiol 2 (8): 621–31. doi:10.1038/nrmicro954. PMID 15263897.
  3. Blankenship RE, Madigan MT & Bauer CE (1995). Anoxygenic photosynthetic bacteria. Dordrecht, The Netherlands, Kluwer Academic.
  4. 4.0 4.1 4.2 Vessey JK, Pawlowski, K and Bergman B (2005). "Root-based N2-fixing symbioses: Legumes, actinorhizal plants, Parasponia sp and cycads". Plant and soil 274 (1–2): 51–78. doi:10.1007/s11104-005-5881-5.
  5. Beckwith, J, Tjepkema, J D, Cashon, R E, Schwintzer, C R, Tisa, L S (2002). "Hemoglobin in five genetically diverse Frankia strains". Can J Microbiol 48 (12): 1048–1055. doi:10.1139/w02-106. PMID 12619816.
  6. 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 USA 92 (7): 2647–2651. doi:10.1073/pnas.92.7.2647. PMC 42275. PMID 7708699.

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