called Zyklon B. Unfortunately, this gas was later used in the genocide of Jewish
prisoners in extermination camps during the Second World War. It is paradoxical
that Haber himself was Jewish. He left Germany in 1933 to avoid Nazi persecution.
The method for large-scale synthesis of ammonia was developed by the German
technologist Carl Bosch (1874–1940). Carl Bosch shared the 1931 Nobel Prize in
Chemistry jointly with Friedrich Bergius for contributions to invention and development of high pressure chemical technology. This direct synthesis of ammonia is
something that no living organism can perform. Plants as well as all microorganisms
receive nitrogen in the form of nitrate ions NO 3
À . Prior to their incorporation into the
organisms’ organic compounds, these ions need to be reduced to ammonia cations
NH 4
+
.
8.4 Fixation of Nitrogen by the Genus Azotobacter, History,
and Description
Azotobacter is one genera of the Azotobacter group which belongs to the family
Pseudomonadaceae, and the resting stage of Azotobacter is a cyst. Another genus in
the Azotobacter group is Azomonas, which does not form cysts. Here we deal with
the Azotobacter only. Its name is derived from the Greek word azoton, meaning
nitrogen. Literally “nitrogen bacterium.” The genus Azotobacter contains gramnegative chemoorganotrophic bacteria, which are obligatory aerobic, ovoid, or in
the shape of roundish rods, size of 2–3 Â 3–5 μm. They do not form spores and
belong to the class Gammaproteobacteria. They display pleomorphism (existence in
a variety of shapes) depending on the living conditions. These bacteria move by the
use of flagella. Only two species, Azotobacter beijerinckii and A. nigricans, are
immobile. These bacteria live freely in the soil, and they can fix gaseous N 2 from the
atmosphere since they contain an enzyme nitrogenase, which is very sensitive to the
action of molecular oxygen. Azotobacter chroococcum represents a typical species
of the genus.
Azotobacter belong to symbiotic fixers of nitrogen and are most often found in
soils which are either pH neutral or slightly alkaline. In soils of pH lower than 6.0,
Azotobacter is almost not present. It also requires soil which is well aerated and
containing a certain degree of moisture. There is a relationship between Azotobacter
and some species of the genus Pseudomonas. There exists a certain type of symbiosis between them, which appears to be quite strong so much so that it is very
difficult to isolate Azotobacter from the soil in the form of a pure culture. In the
symbiotic form, Azotobacter fixes considerably more molecular nitrogen.
Although Azotobacter does not form spores, as mentioned above, it is capable of
developing cysts, which are rigid corpuscules resistant to dryness as well as being
resistant to other adverse conditions of the outside environment. Azotobacter was
discovered in 1901 by a Dutch microbiologist Martinus Willem Beijerinck. For the
isolation of Azotobacter, he used a medium composed of inorganic salts and
8 Miraculous Fixation of Molecular Nitrogen from the Atmosphere
145
prisoners in extermination camps during the Second World War. It is paradoxical
that Haber himself was Jewish. He left Germany in 1933 to avoid Nazi persecution.
The method for large-scale synthesis of ammonia was developed by the German
technologist Carl Bosch (1874–1940). Carl Bosch shared the 1931 Nobel Prize in
Chemistry jointly with Friedrich Bergius for contributions to invention and development of high pressure chemical technology. This direct synthesis of ammonia is
something that no living organism can perform. Plants as well as all microorganisms
receive nitrogen in the form of nitrate ions NO 3
À . Prior to their incorporation into the
organisms’ organic compounds, these ions need to be reduced to ammonia cations
NH 4
+
.
8.4 Fixation of Nitrogen by the Genus Azotobacter, History,
and Description
Azotobacter is one genera of the Azotobacter group which belongs to the family
Pseudomonadaceae, and the resting stage of Azotobacter is a cyst. Another genus in
the Azotobacter group is Azomonas, which does not form cysts. Here we deal with
the Azotobacter only. Its name is derived from the Greek word azoton, meaning
nitrogen. Literally “nitrogen bacterium.” The genus Azotobacter contains gramnegative chemoorganotrophic bacteria, which are obligatory aerobic, ovoid, or in
the shape of roundish rods, size of 2–3 Â 3–5 μm. They do not form spores and
belong to the class Gammaproteobacteria. They display pleomorphism (existence in
a variety of shapes) depending on the living conditions. These bacteria move by the
use of flagella. Only two species, Azotobacter beijerinckii and A. nigricans, are
immobile. These bacteria live freely in the soil, and they can fix gaseous N 2 from the
atmosphere since they contain an enzyme nitrogenase, which is very sensitive to the
action of molecular oxygen. Azotobacter chroococcum represents a typical species
of the genus.
Azotobacter belong to symbiotic fixers of nitrogen and are most often found in
soils which are either pH neutral or slightly alkaline. In soils of pH lower than 6.0,
Azotobacter is almost not present. It also requires soil which is well aerated and
containing a certain degree of moisture. There is a relationship between Azotobacter
and some species of the genus Pseudomonas. There exists a certain type of symbiosis between them, which appears to be quite strong so much so that it is very
difficult to isolate Azotobacter from the soil in the form of a pure culture. In the
symbiotic form, Azotobacter fixes considerably more molecular nitrogen.
Although Azotobacter does not form spores, as mentioned above, it is capable of
developing cysts, which are rigid corpuscules resistant to dryness as well as being
resistant to other adverse conditions of the outside environment. Azotobacter was
discovered in 1901 by a Dutch microbiologist Martinus Willem Beijerinck. For the
isolation of Azotobacter, he used a medium composed of inorganic salts and
8 Miraculous Fixation of Molecular Nitrogen from the Atmosphere
145