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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]
Contents
- 1 Types of diazotrophs
- 1.1 Free-living diazotrophs
- 1.2 Symbiotic diazotrophs
- 2 Importance
- 3 References
- 4 External links
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
- 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.[1]
- 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.[1]
- 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.[1]
- Oxygenic 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).[1]
- Anoxygenic photosynthetic bacteria do not generate oxygen during photosynthesis, having only a single photosystem which cannot split water. Nitrogenase is expressed under nitrogen limitation. Normally, the expression is regulated via negative feedback from the produced ammonium ion but in the absence of N2, the product is not formed, and the by-product H2 continues unabated [Biohydrogen]. Example species: Rhodobacter sphaeroides, Rhodopseudomonas palustris, Rhodobacter capsulatus.[3]
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.[1]
- Frankias—much less is known about these 'actinorhizal' nitrogen fixers. The bacteria also infect the roots leading to the formation of nodules. Actinorhizal nodules consist of several lobes, each lobe has a similar structure as a lateral root. Frankia is able to colonize the cortical tissue of nodules where it fixes nitrogen.[4] Actinorhizal plants and Frankias also produce hemoglobins,[5] but their role is less well established than for rhizobia.[4] Although at first it appeared that they inhabit 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.[6][4] These footnotes suggest the ontogeny of these replicates rather than the phylogeny. In other words, an ancient gene (from before the angiosperms and gymnosperms diverged) that is unused in most species was reawakened and reused in these species.
- Cyanobacteria—there are also symbiotic cyanobacteria. Some associate with fungi as lichens, with liverworts, with a fern, and with a cycad.[1] 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.[1]
- Association with animals—although diazotrophs have been found in many animal guts, there is usually sufficient ammonia present to suppress nitrogen fixation.[1] 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.[1]
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
- ^ a b c d e f g h i j k l Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK.
- ^ 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.
- ^ Blankenship RE, Madigan MT & Bauer CE (1995). Anoxygenic photosynthetic bacteria. Dordrecht, The Netherlands, Kluwer Academic.
- ^ a b c 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.
- ^ 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.
- ^ 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.
External links
- An introduction to Nitrogen fixation by marine diazotrophs (USC Capone Lab)
- Azotobacter
- Rhizobia
- Frankia & Actinorhizal Plants
English Journal
- Whole-genome analysis of Azoarcus sp. strain CIB provides genetic insights to its different lifestyles and predicts novel metabolic features.
- Martín-Moldes Z1, Zamarro MT1, Del Cerro C1, Valencia A1, Gómez MJ2, Arcas A2, Udaondo Z1, García JL1, Nogales J1, Carmona M1, Díaz E3.
- Systematic and applied microbiology.Syst Appl Microbiol.2015 Jul 22. pii: S0723-2020(15)00113-7. doi: 10.1016/j.syapm.2015.07.002. [Epub ahead of print]
- The genomic features of Azoarcus sp. CIB reflect its most distinguishing phenotypes as a diazotroph, facultative anaerobe, capable of degrading either aerobically and/or anaerobically a wide range of aromatic compounds, including some toxic hydrocarbons such as toluene and m-xylene, as well as its e
- PMID 26259823
- Aerobic Hydrogen Production via Nitrogenase in Azotobacter vinelandii CA6.
- Noar J1, Loveless T2, Navarro-Herrero JL3, Olson JW4, Bruno-Bárcena JM5.
- Applied and environmental microbiology.Appl Environ Microbiol.2015 Jul;81(13):4507-16. doi: 10.1128/AEM.00679-15. Epub 2015 Apr 24.
- The diazotroph Azotobacter vinelandii possesses three distinct nitrogenase isoenzymes, all of which produce molecular hydrogen as a by-product. In batch cultures, A. vinelandii strain CA6, a mutant of strain CA, displays multiple phenotypes distinct from its parent: tolerance to tungstate, impaired
- PMID 25911479
- Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume.
- Hilton JA1, Satinsky BM2, Doherty M3, Zielinski B4, Zehr JP1.
- The ISME journal.ISME J.2015 Jul;9(7):1557-69. doi: 10.1038/ismej.2014.240. Epub 2014 Dec 16.
- Biological N2 fixation is an important nitrogen source for surface ocean microbial communities. However, nearly all information on the diversity and gene expression of organisms responsible for oceanic N2 fixation in the environment has come from targeted approaches that assay only a small number of
- PMID 25514535
Japanese Journal
- Phosphate release due to excess alkaline phosphatase activity in <i>Trichodesmium erythraeum</i>
- Why is Trichodesmium abundant in the Kuroshio?
- Widespread Distribution and Expression of Gamma A (UMB), an Uncultured, Diazotrophic, γ-Proteobacterial nifH Phylotype
Related Links
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