Azotobacter

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Azotobacter
Azotobacter

Azotobacter

Systematics
Domain : Bacteria (bacteria)
Department : Proteobacteria
Class : Gammaproteobacteria
Order : Pseudomonadales
Family : Pseudomonadaceae
Genre : Azotobacter
Scientific name
Azotobacter
Beijerinck 1901

Azotobacter is a genus of Gram-negative bacteria that belongs to the gamma group of proteobacteria . A special property is the diazotrophy , i. H. the ability to use elemental nitrogen (diazotogen, N 2 ) as a nitrogen source for growth. In contrast to many other diazotrophic bacteria, members of the genus Azotobacterare capable of thiseven in the presence of molecular oxygen, i.e. under aerobic conditions.

As the first bacterium of this genus, Azotobacter chroococcum was described by Martinus Willem Beijerinck in 1901 .

properties

Appearance

Azotobacter cells are relatively large with a diameter of up to 4 µm. Most species are rod-shaped, but pleomorphism is common ; That is, the cell shape can vary depending on the environment or cell age. In young cultures, for example, rod-shaped cells can be found that occur singly or in pairs; in older cultures, however, the cells are more spherical ( coke-shaped ). Short cell chains can also occur. Some species are flagellated peritrich and thereby capable of active movement. Azotobacter cells can form permanent stages ( cysts ), which are resistant to dehydration and UV, but - unlike the endospores of other bacteria - are sensitive to heat.

Some species secrete water-soluble dyes.

Metabolism and nitrogen fixation

The species of the genus Azotobacter are obligate aerobic , i. that is, they can only metabolize and multiply in the presence of oxygen. Many different carbohydrates , alcohols and organic acids are used as a source of energy and building materials. Members of the genus Azotobacter do not ferment , they are nonfermentative. On nutrient media containing carbohydrates, the bacteria form thick layers of mucus ("mucous capsules") made of alginate , which is used biotechnologically.

An important characteristic of Azotobacter is its ability under oxic conditions, i. H. to assimilate in the presence of oxygen , elemental, molecular nitrogen (N 2 ). The nitrogen is reduced to ammonia by the bacteria using a specific enzyme, a nitrogenase . This process is called nitrogen fixation and the bacteria as nitrogen fixer . In the following steps, ammonia is used for the formation of amino acids and thus for the construction of proteins as well as for the synthesis of other nitrogen-containing body building materials. Because of this property, Azotobacter can be cultivated on culture media without nitrogen compounds (e.g. mannitol agar ). On the other hand, Azotobacter is also able to live without N 2 and use simple nitrogen compounds such as nitrates , urea or ammonia as a nitrogen source . Azotobacter can, however, metabolize and grow even under low oxygen concentrations, under these circumstances an increased nitrogen fixation takes place.

The enzyme nitrogenase is used for N 2 fixation . The product is ammonia, which is used to build up amino acids and other nitrogen compounds. Nitrogenase is very sensitive to oxygen. It is therefore surprising that Azotobacter is able to fix N 2 under normal oxygen concentrations. Presumably, the oxygen concentration inside the cell is caused by intensive breathing, i.e. H. kept low by an intensive energy metabolism and a mucous capsule. It is known that nitrogenase requires iron and molybdenum to function . Azotobacter chroococcum was one of the first bacteria in which it was found that so-called alternative nitrogenases, which contain vanadium or even iron instead of molybdenum, are also used in the event of a molybdenum deficiency . These alternative nitrogenases, also found in other bacteria (e.g. Rhodobacter capsulatus ), are only formed when there is a molybdenum deficiency.

ecology

Azotobacter and other nitrogen-fixing bacteria are of great ecological importance because they bind the elementary nitrogen (N 2 ) that cannot be used by eukaryotes and thus reintroduce it into the nitrogen cycle . Bacteria that are able to bind N 2 are called diazotrophic .

Although Azotobacter is one of the free-living nitrogen fixers, and thus does not depend on symbioses like some other diazotrophs to bind N 2 , some species occur more frequently in the vicinity of plant roots ( rhizosphere ) than in the free soil. The plant and the bacterium are not directly dependent on each other for survival, so it is not an obligatory mutualism, one speaks of an associative symbiosis. The plant may benefit from the uptake of the nitrogen compounds formed by nitrogen fixation, while the bacteria in turn benefit from the nutrient-rich environment created by the plant.

Bacteria of this genus are mainly found in neutral or slightly alkaline soils, Azotobacter chroococcum and A. vinelandii also in seawater. Azotobacter chroococcum also lives epiphytically on plants; the bacterium is often found on leaf surfaces. Azotobacter paspali is found on the root surface ( rhizosphere ) of the sweet grass Paspalum . This is probably a very species-specific relationship; the bacterium was only found in a few species of grass (e.g. P. plicatulum ).

Differences from some other nitrogen fixers

Many of the free-living nitrogen fixators can fix N 2 only with a low concentration of oxygen or only in an anoxic environment, i.e. complete exclusion of oxygen . Azomonas is a closely related genus and is also able to bind N 2 under normal oxygen concentrations . In contrast to Azotobacter, bacteria of this genus do not form cysts and usually live in water. Derxia and Beijerinckia are further free-living nitrogen fixers and occur in acidic soils, while Azotobacter prefers a neutral or slightly alkaline environment. In the cell chains of some representatives of the cyanobacteria there are special cells for nitrogen fixation, the so-called heterocysts , which are protected by a thickened cell wall. The well-studied nodule bacteria (rhizobia) are symbiotic nitrogen fixers and can only bind N 2 within cells formed by the plant. The oxygen concentration is kept low by the plant.

etymology

The name consists of two parts: the first refers to the ability of the bacterium, elemental nitrogen (dinitrogen, N 2 ) to use as a nitrogen source: French azote for nitrogen ( Azotogenium ) according to ancient Greek ἀ without and ζωή zoe life . The second part says that Azotobacter is a bacterium : ancient Greek βακτήριον ( bacterion ) rods, shortened to –bacter .

Systematics and synonyms

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

In January 2018 the following species were assigned to the genus:

Some synonyms:

  • Azotobacter macrocytogenes is the basionym of Azomonas macrocytogenes (Jensen 1955) New and Tchan 1982
  • Synonyms for Azotobacter chroococcum : Bacillus chroococcus and Bacillus azotobacter
  • Azotobacter paspali Döbereiner 1966 was assigned to the genus Azorhizophilus , the correct name is now Azorhizophilus paspali (Döbereiner 1966) Thompson and Skerman 1981 . This change was made due to some strong differences to other Azotobacter. In some cases, cell shapes of relatively long rods (60 μm) were observed in nitrogen-free cultures. Furthermore, this species was found exclusively in rhizospheres, in contrast to the other species also living freely in the ground. This type of acid production is also observed in cultures. However, this change has been called into question again due to other work on taxonomy.

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Individual evidence

  1. ↑ NCBI
  2. ↑ JP Euzéby: List of Prokaryotic Names with Standing in Nomenclature - Genus Azotobacter. As of January 24, 2019
  3. ↑ De Smedt, J., Bauwens, M., Tytgat, R., De Ley, J. 1980: Intraand intergeneric similaritis of ribosomal ribonucleic acid cistrons of free-living, nitrogen-fixing bacteria. In: International Journal Systematic Bacteriology , 30: 106-122
  4. ↑ Tchan, YT, Wyszomirska-Dreher, Z., New, PB, Zhou, JC. 1983: Taxonomy of the Azotobacteraceae determined by using immuno-electrophoresis. In: International Journal of Systematic Bacteriology , 33: 147-156.

literature

  • Martin Dworkin, Stanley Falkow, Eugene Rosenberg, Karl-Heinz Schleifer , Erko Stackebrandt (Eds.): The Prokaryotes, A Handbook of the Biology of Bacteria . 7 volumes, 3rd edition, Springer-Verlag, New York et al. O., 2006, ISBN 0-387-30740-0 . Vol. 6: Proteobacteria: Gamma Subclass ISBN 0-387-30746-X .
  • Michael T. Madigan, John M. Martinko, Jack Parker: Brock - Microbiology . 11th edition. Pearson Studium, Munich 2006, ISBN 3-8274-0566-1 .

Web links

Wiktionary: Azotobacter  - explanations of meanings, word origins, synonyms, translations