Azotobacter

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Azotobacter
Azotobacter species cells, stained with Heidenhain's iron hematoxylin, ×1000
Scientific classification Edit this classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Pseudomonadales
Family: Pseudomonadaceae
Genus: Azotobacter
Beijerinck 1901
Species

Azotobacter agilis
Azotobacter armeniacus
Azotobacter beijerinckii
Azotobacter chroococcum
Azotobacter nigricans
Azotobacter salinestris
Azotobacter tropicalis
Azotobacter vinelandii

Azotobacter beijerinckii on agar plate

Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it in the form of ammonium ions into the soil (nitrogen fixation). In addition to being a model organism for studying diazotrophs, it is used by humans for the production of biofertilizers, food additives, and some biopolymers. The first representative of the genus, Azotobacter chroococcum, was discovered and described in 1901 by Dutch microbiologist and botanist Martinus Beijerinck. Azotobacter species are Gram-negative bacteria found in neutral and alkaline soils,[1][2] in water, and in association with some plants.[3][4]

Biological characteristics

Morphology

peptone.[6]

Under magnification, the cells show inclusions, some of which are colored. In the early 1900s, the colored inclusions were regarded as "reproductive grains", or

cell division.[8] The colored grains are composed of volutin, whereas the colorless inclusions are drops of fat, which act as energy reserves.[9]

Cysts

Cysts of the genus Azotobacter are more resistant to adverse environmental factors than the

solar irradiation, but not to heating.[10]

The formation of cysts is induced by changes in the concentration of nutrients in the medium and addition of some organic substances such as

β-hydroxybutyrate. Cysts are rarely formed in liquid media.[11] The formation of cysts is induced by chemical factors and is accompanied by metabolic shifts, changes in catabolism, respiration, and biosynthesis of macromolecules;[12] it is also affected by aldehyde dehydrogenase[13] and the response regulator AlgR.[14]

The cysts of Azotobacter are spherical and consist of the so-called "central body" – a reduced copy of vegetative cells with several

viable state by some chelation agents.[18] The main constituents of the outer shell are alkylresorcinols composed of long aliphatic chains and aromatic rings. Alkylresorcinols are also found in other bacteria, animals, and plants.[19]

Germination of cysts

A cyst of the genus Azotobacter is the resting form of a

phase contrast microscopy. Germination of cysts takes about 4–6 h. During germination, the central body grows and captures the granules of volutin, which were located in the intima (the innermost layer). Then, the exine bursts and the vegetative cell is freed from the exine, which has a characteristic horseshoe shape.[20] This process is accompanied by metabolic changes. Immediately after being supplied with a carbon source, the cysts begin to absorb oxygen and emit carbon dioxide; the rate of this process gradually increases and saturates after four hours. The synthesis of proteins and RNA occurs in parallel, but it intensifies only after five hours after the addition of the carbon source. The synthesis of DNA and nitrogen fixation are initiated 5 hours after the addition of glucose to a nitrogen-free nutrient medium.[21]

Germination of cysts is accompanied by changes in the intima, visible with an electron microscope. The intima consists of

carbohydrates, lipids, and proteins and has almost the same volume as the central body. During germination of cysts, the intima hydrolyses and is used by the cell for the synthesis its components.[22]

Physiological properties

Azotobacter

respires aerobically, receiving energy from redox reactions, using organic compounds as electron donors, and can use a variety of carbohydrates, alcohols, and salts of organic acids
as sources of carbon.

Azotobacter can fix at least 10 μg of nitrogen per gram of glucose consumed. Nitrogen fixation requires molybdenum ions, but they can be partially or completely replaced by vanadium ions. If atmospheric nitrogen is not fixed, the source of nitrogen can alternatively be nitrates, ammonium ions, or amino acids. The optimal pH for the growth and nitrogen fixation is 7.0–7.5, but growth is sustained in the pH range from 4.8 to 8.5.[23] Azotobacter can also grow mixotrophically, in a molecular nitrogen-free medium containing mannose; this growth mode is hydrogen-dependent. Hydrogen is available in the soil, thus this growth mode may occur in nature.[24]

While growing, Azotobacter produces flat, slimy, paste-like colonies with a diameter of 5–10 mm, which may form films in liquid nutrient media. The colonies can be dark-brown, green, or other colors, or may be colorless, depending on the species. The growth is favored at a temperature of 20–30°C.[25]

Bacteria of the genus Azotobacter are also known to form intracellular inclusions of polyhydroxyalkanoates under certain environmental conditions (e.g. lack of elements such as phosphorus, nitrogen, or oxygen combined with an excessive supply of carbon sources).

Pigments

Azotobacter produces pigments. For example, Azotobacter chroococcum forms a dark-brown water-soluble pigment melanin. This process occurs at high levels of metabolism during the fixation of nitrogen, and is thought to protect the nitrogenase system from oxygen.[26] Other Azotobacter species produce pigments from yellow-green to purple colors,[27] including a green pigment which fluoresces with a yellow-green light and a pigment with blue-white fluorescence.[28]

Genome

The nucleotide sequence of chromosomes of Azotobacter vinelandii, strain AvOP, is partially determined. This chromosome is a circular DNA molecule which contains 5,342,073 nucleotide pairs and 5,043 genes, of which 4,988 encode proteins. The fraction of guanine + cytosine pairs is 65 mole percent. The number of chromosomes in the cells and the DNA content increases upon aging, and in the stationary growth phase, cultures may contain more than 100 copies of a chromosome per cell. The original DNA content (one copy) is restored when replanting the culture into a fresh medium.[29] In addition to chromosomal DNA, Azotobacter can contain plasmids.[30]

Distribution

Azotobacter species are ubiquitous in

basic soils, but not acidic soils.[31] They are also found in the Arctic and Antarctic soils, despite the cold climate, short growing season, and relatively low pH values of these soils.[32] In dry soils, Azotobacter can survive in the form of cysts for up to 24 years.[33]

Representatives of the genus Azotobacter are also found in aquatic habitats, including fresh water[34] and brackish marshes.[35] Several members are associated with plants and are found in the rhizosphere, having certain relationships with the plants.[36] Some strains are also found in the cocoons of the earthworm Eisenia fetida.[37]

Nitrogen fixation

Main article: Nitrogen fixation

Azotobacter species are free-living, nitrogen-fixing bacteria; in contrast to Rhizobium species, they normally fix molecular nitrogen from the atmosphere without symbiotic relations with plants, although some Azotobacter species are associated with plants.[38] Nitrogen fixation is inhibited in the presence of available nitrogen sources, such as ammonium ions and nitrates.[39]

Azotobacter species have a full range of enzymes needed to perform the nitrogen fixation: ferredoxin, hydrogenase, and an important enzyme nitrogenase. The process of nitrogen fixation requires an influx of energy in the form of adenosine triphosphate. Nitrogen fixation is highly sensitive to the presence of oxygen, so Azotobacter developed a special defensive mechanism against oxygen, namely a significant intensification of metabolism that reduces the concentration of oxygen in the cells.[40] Also, a special nitrogenase-protective protein protects nitrogenase and is involved in protecting the cells from oxygen. Mutants not producing this protein are killed by oxygen during nitrogen fixation in the absence of a nitrogen source in the medium.[41] Homocitrate ions play a certain role in the processes of nitrogen fixation by Azotobacter.[42]

Nitrogenase

Main article: Nitrogenase

Nitrogenase is the most important enzyme involved in nitrogen fixation. Azotobacter species have several types of nitrogenase. The basic one is molybdenum-iron nitrogenase.[43] An alternative type contains vanadium; it is independent of molybdenum ions[44][45][46] and is more active than the Mo-Fe nitrogenase at low temperatures. So it can fix nitrogen at temperatures as low as 5 °C, and its low-temperature activity is 10 times higher than that of Mo-Fe nitrogenase.[47] An important role in maturation of Mo-Fe nitrogenase plays the so-called P-cluster.[48] Synthesis of nitrogenase is controlled by the nif genes.[49] Nitrogen fixation is regulated by the enhancer protein NifA and the "sensor" flavoprotein NifL which modulates the activation of gene transcription of nitrogen fixation by redox-dependent switching.[50] This regulatory mechanism, relying on two proteins forming complexes with each other, is uncommon for other systems.[51]

Importance

Nitrogen fixation plays an important role in the nitrogen cycle. Azotobacter also synthesizes some biologically active substances, including some

2,4,6-trichlorophenol, which was previously used as an insecticide, fungicide, and herbicide, but later was found to have mutagenic and carcinogenic effects.[56]

Applications

Owing to their ability to fix molecular nitrogen and therefore increase the soil fertility and stimulate plant growth, Azotobacter species are widely used in agriculture,[57] particularly in nitrogen biofertilizers such as azotobacterin. They are also used in production of alginic acid,[58][59][60] which is applied in medicine as an antacid, in the food industry as an additive to ice cream, puddings, and creams.[61]

Taxonomy

Martinus Beijerinck (1851–1931), discoverer of the genus Azotobacter

The genus Azotobacter was discovered in 1901 by Dutch microbiologist and botanist Martinus Beijerinck, who was one of the founders of

environmental microbiology. He selected and described the species Azotobacter chroococcum – the first aerobic, free-living nitrogen fixer.[62]

In 1909, Lipman described

microaerophilic and air-tolerant type Azotobacter salinestris Page and Shivprasad 1991 which was dependent on sodium ions.[63]

Earlier, representatives of the genus were assigned to the family Azotobacteraceae Pribram, 1933, but then were transferred to the family Pseudomonadaceae based on the studies of nucleotide sequences 16S rRNA. In 2004, a phylogenetic study revealed that A. vinelandii belongs to the same clade as the bacterium Pseudomonas aeruginosa,[64] and in 2007 it was suggested that the genera Azotobacter, Azomonas and Pseudomonas are related and might be synonyms.[65]

References

  1. ^ Gandora V.; Gupta R. D.; Bhardwaj K. K. R. (1998). "Abundance of Azotobacter in great soil groups of North-West Himalayas". Journal of the Indian Society of Soil Science. 46 (3): 379–383. Archived from the original on 2012-09-06. Retrieved 2010-08-30.
  2. ^ Martyniuk S.; Martyniuk M. (2003). "Occurrence of Azotobacter Spp. in Some Polish Soils" (PDF). Polish Journal of Environmental Studies. 12 (3): 371–374. Archived from the original (PDF) on 2011-07-15. Retrieved 2010-08-30.
  3. S2CID 20917484. Archived
    (PDF) from the original on 2016-03-03. Retrieved 2010-08-30.
  4. S2CID 30574266
    .
  5. doi:10.1111/j.1365-2672.1962.tb01126.x
    .
  6. PMID 4591479
    .
  7. PMID 16558880
    .
  8. PMID 16350071
    .
  9. PMID 16560046
    .
  10. PMID 13914732
    .
  11. PMID 14127586
    .
  12. PMID 1212151
    .
  13. PMID 11591659
    .
  14. PMID 9864323
    .
  15. PMID 4191240
    .
  16. PMID 235508
    .
  17. PMID 4977988
    .
  18. PMID 4955249
    .
  19. PMID 16597676
    .
  20. PMID 13787014
    .
  21. PMID 4690966
    .
  22. PMID 681284
    .
  23. ISBN 978-0-387-95040-2. Archived
    from the original on 2010-01-31. Retrieved 2017-08-28.
  24. PMID 4019408
    .
  25. ^ Tepper EZ, Shilnikova VK, Pereverzev, GI (1979). Workshop on Microbiology. M. p. 216.{{cite book}}: CS1 maint: location missing publisher (link)
  26. PMID 16347974
    .
  27. PMID 13219046
    .
  28. PMID 14367310
    .
  29. PMID 8021173
    .
  30. PMID 3350795
    .
  31. PMID 16559016
    .
  32. PMID 16561931
    .
  33. PMID 16346962
    .
  34. JSTOR 1936516
    .
  35. PMID 16345564
    .
  36. PMID 6775181
    .
  37. PMID 16348968
    .
  38. doi:10.2136/sssaj1971.03615995003500020031x
    .
  39. PMID 12676666
    .
  40. ^ Shank Yu; Demin O.; Bogachev AV (2005). "Respiratory Protection nitrogenase complex in Azotobacter vinelandii" (PDF). Success Biological Chemistry (Sat). 45: 205–234. Archived from the original (PDF) on 2011-07-22. Retrieved 2010-08-30.
  41. PMID 10851006
    .
  42. PMID 16566750
    .
  43. PMID 17088547. Archived
    (PDF) from the original on 2017-12-02. Retrieved 2018-11-04.
  44. PMID 18192412
    .
  45. PMID 12644512
    .
  46. S2CID 4368841
    .
  47. PMID 3223922
    .
  48. PMID 17563349
    .
  49. PMID 15845763
    .
  50. PMID 8700899
    .
  51. PMID 11157949
    .
  52. ^ Ahmad F.; Ahmad I.; Khan M. S. (2005). "Indole Acetic Acid Production by the Indigenous Isolates of Azotobacter and Fluorescent Pseudomonas in the Presence and Absence of Tryptophan" (PDF). Turkish Journal of Biology (29): 29–34. Archived from the original (PDF) on 2010-04-15.
  53. doi:10.17660/actahortic.1985.108.7. Archived
    from the original on 2008-12-02. Retrieved 2010-08-30.
  54. ^ Rajaee S.; Alikhani H. A.; Raiesi F. (2007). "Effect of Plant Growth Promoting Potentials of Azotobacter chroococcum Native Strains on Growth, Yield and Uptake of Nutrients in Wheat". Journal of Science and Technology of Agriculture and Natural Resources. 11 (41): 297. Archived from the original on 2014-02-20. Retrieved 2013-03-22. PDF copy Archived 2018-12-15 at the Wayback Machine
  55. PMID 7621800
    .
  56. PMID 1892382
    .
  57. ISBN 978-81-239-0661-4. Archived
    from the original on 2008-11-20. Retrieved 2010-08-30.
  58. PMID 17306024
    .
  59. PMID 11158365
    .
  60. PMID 18645604
    .
  61. ISBN 978-0-521-43980-0
    .
  62. ^ Beijerinck M. W. (1901). "Ueber Oligonitrophile Mikroben". Zentralblatt für Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene, Abteilung II (in German) (7): 561–582.
  63. doi:10.1099/00207713-41-3-369
    .
  64. PMID 15133068
    .
  65. PMID 18048745
    .

External links
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