Azotobacter

Azotobacter

Azotobacter is a genus of Gram-negative bacteria belonging to the gamma group of proteobacteria. Azotobacter cells are relatively large, having up to 4 microns in diameter. Most species are rod-shaped, but it often occur Pleomorphien on, that is, the cell shape may vary depending on environment or cell age vary. Thus, in young cultures rod-shaped cells to find which occur singly or in pairs; in older cultures the cells, however, are rather kokkenförmig. Even short chains of cells may occur. Some species are flagellated and peritrichous thereby able to exercise. Azotobacter cells can form resting stages ( cysts ) which desiccation and UV - resistant, but - unlike endospores - are sensitive to heat. Some species but water-soluble dyes from.

An important feature is the ability of Azotobacter under oxic conditions, ie in the presence of oxygen, elemental molecular nitrogen (N2) to assimilate. The nitrogen is in this case reduced by the bacteria by a specific enzyme, a nitrogenase, to form ammonia. This process is known as nitrogen fixation. In the following steps, ammonia is used body building materials for the formation of amino acids and thus for the synthesis of proteins and for the synthesis of other nitrogen -containing.

The first bacterium Azotobacter chroococcum 1901 this genus was described by Martinus Willem Beijerinck in years.

Metabolism and nitrogen fixation

The species of the genus Azotobacter can live only in the presence of oxygen, they are obligately aerobic. Many different carbohydrates, alcohols and organic acids are used as a food source. Azotobacter do not perform fermentation, they are nonfermentativ. The bacteria form on carbohydrate media containing thick mucus capsules of alginate, which is used biotechnologically. Since Azotobacter fixed elemental, molecular nitrogen, it can be on culture media without nitrogen compounds cultivate (eg Mannitagar ). It is on the other hand able to live without N2 and utilize simple nitrogen compounds such as nitrates, urea or ammonia. However, Azotobacter can metabolize even under low oxygen concentrations and grow under these circumstances is an increased nitrogen fixation instead.

For the N2 - fixing enzyme nitrogenase that is being used. As the product gives rise to ammonia, which is used for the construction of amino acids and other nitrogen compounds. Nitrogenase is highly sensitive to oxygen. Therefore, it is surprising that Azotobacter is able to fix N2 under normal oxygen concentrations. Presumably, the oxygen concentration within the cell by an intense respiration, that is, by an intensive energy metabolism, as well as by a mucus capsule kept low. It is known that the function required nitrogenase iron and molybdenum. Azotobacter chroococcum was one of the first bacteria was shown for that case of molybdenum deficiency, so-called alternative nitrogenases are used which contain vanadium instead of molybdenum or even iron. This also in other bacteria (eg Rhodobacter capsulatus ) found alternative nitrogenases are formed only under molybdenum deficiency.

Ecology

Azotobacter and other nitrogen-fixing bacteria are of great ecological importance because they bind the non-usable for eukaryotes elemental nitrogen (N2) and thus reintroduce in the nitrogen cycle. Bacteria which are able to bind N2 are referred to as diazotroph.

Although Azotobacter is one of the free-living diazotrophs, and thus does not rely on symbioses to bind N2, we find some species more common in the vicinity of plant roots ( rhizosphere ) than in the free soil. The plant and the bacteria are not directly dependent on each other for survival, so it is not it is an obligate mutualism, we speak here of an associative symbiosis. May benefit the plant by absorbing the fixed nitrogen, the bacteria turn of the jobs created by the plant nutrient- rich environment.

One finds bacteria of this genus mainly in neutral or slightly alkaline soils, Azotobacter chroococcum and A. vinelandii in the sea water. Azotobacter chroococcum also lives on epiphytic plants, one finds the bacterium frequently on leaf surfaces. Azotobacter paspali can be found on the root surface ( rhizosphere ) of sweet grass Paspalum. This is likely to be a strong relationship artspezische, the bacterium was only a few species of grass found ( eg P. plicatulum ).

Differences to some other diazotrophs

Many of the free-living nitrogen fixers can fix N2 only at low concentrations of oxygen only or in an anoxic environment, ie complete exclusion of oxygen. Azomonas is a closely related species, and is also able to bind under normal oxygen concentration N2. However, bacteria of this genus form unlike any Azotobacter cysts and usually live in water. Derxia and Beijerinckia are more free-living nitrogen fixers and occur in acidic soils on, Azotobacter, however, prefers neutral or slightly alkaline environment. In the cell chains of some representatives of cyanobacteria are found special cells for nitrogen fixation, heterocysts so called, which are protected by a thickened cell wall. The well-studied nodule bacteria ( rhizobia ) are symbiotic nitrogen fixers and N2 may only within the plant cells formed bind. Here, the oxygen concentration is kept low by the plant.

Systematics and Synonyms

Some authors Azotobacter and Azomonas also as members of the Azotobacteraceae family (also written with i: Azotobacteriaceae ) viewed. Furthermore, the genera Azotobacter, Azorhizophilus and Azomonas by some authors in the Azotobacter group ( Azotobacter group) are summarized.

The following types are known:

  • Azotobacter armeniacus Thompson and Skerman 1981
  • Azotobacter beijerinckii Lipman 1904
  • Azotobacter chroococcum Beijerinck 1901
  • Azotobacter macrocytogenes Jensen 1955
  • Azotobacter nigricans subsp. achromogenes Thompson and Skerman 1981
  • Azotobacter nigricans subsp. nigricans Krassilnikov 1949
  • Azotobacter salinestris Page and Shivprasad 1991
  • Azotobacter vinelandii Lipman 1903

Some synonyms:

  • Azotobacter macrocytogenes the basionym of Azomonas macrocytogenes ( Jensen 1955) New and Tchan in 1982
  • Synonyms for Azotobacter chroococcum: Chroococcus Bacillus and Bacillus Azotobacter
  • Azotobacter paspali Dobereiner 1966 was provided to the genus Azorhizophilus, the correct name is now Azorhizophilus paspali ( Dobereiner 1966) Thompson and Skerman 1981. This change was due to some strong differences with other Azotobactern. Partial cell lines were observed by relatively long rods (60 microns ) in nitrogen-free cultures. Furthermore, this type was exclusively in the rhizosphere, found in contrast to the other species also free living in the soil. In addition, we observed in cultures of this type acid production. This change, however, was called into question again because of other work on the taxonomy.

Swell

94271
de