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).

Types of diazotrophs

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, 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).

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, 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).

Importance

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).

References

  • 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: 1048-1055. 
  • Dixon R and Kahn D (2004). "Genetic regulation of biological nitrogen fixation". Nat Rev Microbiol 2 (8): 621-31. 
  • Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK. 
  • 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: 2647 – 2651. 
  • 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. 

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Bacteria

Phyla

Actinobacteria
Aquificae
Chlamydiae
Bacteroidetes/Chlorobi
Chloroflexi
Chrysiogenetes
Cyanobacteria
Deferribacteres
Deinococcus-Thermus
Dictyoglomi
Fibrobacteres/Acidobacteria
Firmicutes
Fusobacteria
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Nitrogen fixation is the process by which nitrogen is taken from its natural, relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds (such as, notably, ammonia, nitrate and nitrogen dioxide)[1]
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3, 5, 4, 2
(strongly acidic oxide)
Electronegativity 3.04 (Pauling scale)
Ionization energies
(more) 1st: 1402.3 kJmol−1
2nd: 2856 kJmol−1
3rd: 4578.1 kJmol−1

Atomic radius 65 pm
Atomic radius (calc.
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Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers.
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Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia cannot independently fix nitrogen, and require a plant host.
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Frankiaceae

Genus: Frankia
Brunchorst 1886

Species

F. alni

This article refers to the bacteria. Frankia was also one of the names of the Frankish Empire.

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Nitrogenase (EC 1.18.6.1 ) is the enzyme used by some organisms to fix atmospheric nitrogen gas (N2). It is the only known family of enzymes which accomplishes this process.
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Klebsiella is a genus of non-motile, Gram-negative bacteria with a prominent polysaccharide-based capsule.[1] Frequent human pathogens, Klebsiella
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Azotobacter

Species

Azotobacter vinelandii
Azotobacter chroococcum

Azotobacter is a
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Bacteria

Phyla

Actinobacteria
Aquificae
Chlamydiae
Bacteroidetes/Chlorobi
Chloroflexi
Chrysiogenetes
Cyanobacteria
Deferribacteres
Deinococcus-Thermus
Dictyoglomi
Fibrobacteres/Acidobacteria
Firmicutes
Fusobacteria
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Archaea
Woese, Kandler & Wheelis, 1990

Phyla

Crenarchaeota
Euryarchaeota
Korarchaeota
Nanoarchaeota
ARMAN
The Archaea (
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Clostridium
Prazmowski 1880

Species

C. acetobutylicum
C. aerotolerans
C. botulinum
C. butyricum
C. colicanis
C. difficile
C. formicaceticum
C.
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Desulfovibrio
Kluyver & van Niel 1936

Species

D. desulfuricans
D. gigas
D. salixigens
D. vulgaris
etc.

Desulfovibrio is a genus of Gram negative sulfate-reducing bacteria.
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Klebsiella pneumoniae
Classification & external resources

ICD-10 B 96.1 , G 00.8 , J 15.0 , P 23.6
ICD-9 041.3 , 320.82 , 482.0

DiseasesDB 7181

eMedicine med/1237  

MeSH C01.252.400.310.
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Azotobacter vinelandii

Azotobacter vinelandii is a diazotroph that can fix nitrogen while grown aerobically. It is a genetically tractable system that is used to study nitrogen fixation.
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Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale and Anabaena sperica, during nitrogen starvation.
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Cyanobacteria

Orders

The taxonomy is currently under revision.[1]

Cyanobacteria (Greek: κυανόs (kyanós) = blue + bacterium) also known as Cyanophyta
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Rhizobia (from the Greek words rhiza = root and bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). The rhizobia cannot independently fix nitrogen, and require a plant host.
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Fabaceae
Lindl.

Subfamilies

Caesalpinioideae
Mimosoideae
Faboideae
References

GRIN-CA 2002-09-01

The Fabaceae or Leguminosae
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Frankiaceae

Genus: Frankia
Brunchorst 1886

Species

F. alni

This article refers to the bacteria. Frankia was also one of the names of the Frankish Empire.

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Alnus
Mill.

Species

About 20-30 species, see text.

Alder is the common name of a genus of flowering plants (Alnus) belonging to the birch family (Family Betulaceae).
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Casuarina can refer to:
  • Casuarina, a genus of plants from the family Casuarinaceae.
  • Casuarina, Northern Territory
  • Casuarina, Western Australia
  • Casuarina Prison, a prison in Western Australia.
  • Casuarina, Brazilian music group

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Ceanothus
L.

Species
See text

Ceanothus L. is a genus of about 50–60 species of shrubs or small trees in the buckthorn family Rhamnaceae.
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M. gale

Binomial name
Myrica gale
L.

Myrica gale is a species of flowering plant in the genus Myrica, native to northern and western Europe and parts of northern North America.
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Purshia
Spreng.

Species
See text

Purshia (bitterbrush or cliff-rose) is a small genus of 5-8 species of flowering plants in the family Rosaceae, native to western North America, where they grow in dry climates from
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Dryas
L.

Speciea
See text

Dryas is a genus of dwarf perennial herbaceous plants in the rose family Rosaceae, native to the arctic and alpine regions of Europe, Asia and North America. The genus is named after the Greek nymph Dryas.
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phylogenetics (Greek: phyle = tribe, race and genetikos = relative to birth, from genesis = birth) is the study of evolutionary relatedness among various groups of organisms (e.g., species, populations).
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Magnoliophyta

Classes

Magnoliopsida - Dicots
Liliopsida - Monocots

The flowering plants or angiosperms are the most widespread group of land plants. The flowering plants and the gymnosperms comprise the two extant groups of seed plants.
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Cyanobacteria

Orders

The taxonomy is currently under revision.[1]

Cyanobacteria (Greek: κυανόs (kyanós) = blue + bacterium) also known as Cyanophyta
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