are depleted. The cysts are not considered to be spores because their interiors
resemble vegetative cells although their metabolic activity is greatly reduced.
Also, they are not thermoresistant; however they are resistant to the influence of
chemical factors. In a dry soil, they remain viable for more than 10 years. In the
1960s, there were reports claiming the cysts can remain viable up to 2300 years.
Morphologically, the cyst contains the central body which resembles a vegetative
cell. Then there is a cytoplasmic membrane and a cellular wall composed of muramic
acid. In addition, there are two layers of polysaccharides and lipids called intina and
exina, both of which contain a highly hydrated alginate, which prevents drying of the
cyst. The biosynthesis of alginate is essential for development of the cyst. Mutants
unable to form alginate are also unable to differentiate into cysts. The factor AlgU is
responsible for the production of alginate. In the cysts, the lipid content is twice as
high compared to vegetative cells. Seventy percent of the cyst lipid content is
localized in the central body and 25 percent in the exina. The cyst formation can
also be induced either by growth of the vegetative cells in medium containing
n-butanol or by a transfer of the culture into medium that contains
polyhydroxybutyrate.
8.4.2 Nitrogenase
Bacteria-fixing atmospheric nitrogen from the air must contain the enzyme nitrogenase with iron as a prosthetic group which activates nitrogen. Originally it was
thought that nitrogenase is just one enzyme. However it has been later established
that it is what we now call a “nitrogenase complex.”
Nitrogenase is composed of two parts, neither of which has any function by itself.
One of the two components has a molecular mass of 2.4 Â 10
5 g/mol. Its molecule
contains two atoms of molybdenum and 28–32 atoms of non-heme iron which is
bound to sulfur. This component is called Mo-Fe protein, or molybdoferredoxin
(also dinitrogenase or component 1). It reduces gaseous nitrogen to ammonia. In
some cases, molybdenum can be replaced by vanadium. The other component is
more acidic and has a lower molecular mass of 6 Â 10
4 g/ml. It contains two atoms
of iron, as well as two SH groups. It is called Fe protein, or azoferredoxin (component 2). It reduces dinitrogenase. Apart from nitrogen and nitrogenase, a source of
hydrogen as well as a source of energy in the form of ATP is also essential for the
fixation of nitrogen. Intermediate products of the microbial synthesis of ammonia
include compounds called hydrazine N 2 H 4 and hydroxylamine NH 2 OH. Nitrogenase is sensitive to molecular oxygen, which inactivates it. Therefore a certain
protection from oxygen is necessary.
In the case of cyanobacteria, for example, this protection is achieved such that the
nitrogen fixation takes place in specialized non-photosynthesizing cells called heterocysts. These are present during the whole vegetative period. They do not contain
any assimilatory pigments. In the bacteria of legumes, oxygen protection is taken
care of by the symbiotic synthesis of so-called leghemoglobin. The globin part of
8 Miraculous Fixation of Molecular Nitrogen from the Atmosphere
147
resemble vegetative cells although their metabolic activity is greatly reduced.
Also, they are not thermoresistant; however they are resistant to the influence of
chemical factors. In a dry soil, they remain viable for more than 10 years. In the
1960s, there were reports claiming the cysts can remain viable up to 2300 years.
Morphologically, the cyst contains the central body which resembles a vegetative
cell. Then there is a cytoplasmic membrane and a cellular wall composed of muramic
acid. In addition, there are two layers of polysaccharides and lipids called intina and
exina, both of which contain a highly hydrated alginate, which prevents drying of the
cyst. The biosynthesis of alginate is essential for development of the cyst. Mutants
unable to form alginate are also unable to differentiate into cysts. The factor AlgU is
responsible for the production of alginate. In the cysts, the lipid content is twice as
high compared to vegetative cells. Seventy percent of the cyst lipid content is
localized in the central body and 25 percent in the exina. The cyst formation can
also be induced either by growth of the vegetative cells in medium containing
n-butanol or by a transfer of the culture into medium that contains
polyhydroxybutyrate.
8.4.2 Nitrogenase
Bacteria-fixing atmospheric nitrogen from the air must contain the enzyme nitrogenase with iron as a prosthetic group which activates nitrogen. Originally it was
thought that nitrogenase is just one enzyme. However it has been later established
that it is what we now call a “nitrogenase complex.”
Nitrogenase is composed of two parts, neither of which has any function by itself.
One of the two components has a molecular mass of 2.4 Â 10
5 g/mol. Its molecule
contains two atoms of molybdenum and 28–32 atoms of non-heme iron which is
bound to sulfur. This component is called Mo-Fe protein, or molybdoferredoxin
(also dinitrogenase or component 1). It reduces gaseous nitrogen to ammonia. In
some cases, molybdenum can be replaced by vanadium. The other component is
more acidic and has a lower molecular mass of 6 Â 10
4 g/ml. It contains two atoms
of iron, as well as two SH groups. It is called Fe protein, or azoferredoxin (component 2). It reduces dinitrogenase. Apart from nitrogen and nitrogenase, a source of
hydrogen as well as a source of energy in the form of ATP is also essential for the
fixation of nitrogen. Intermediate products of the microbial synthesis of ammonia
include compounds called hydrazine N 2 H 4 and hydroxylamine NH 2 OH. Nitrogenase is sensitive to molecular oxygen, which inactivates it. Therefore a certain
protection from oxygen is necessary.
In the case of cyanobacteria, for example, this protection is achieved such that the
nitrogen fixation takes place in specialized non-photosynthesizing cells called heterocysts. These are present during the whole vegetative period. They do not contain
any assimilatory pigments. In the bacteria of legumes, oxygen protection is taken
care of by the symbiotic synthesis of so-called leghemoglobin. The globin part of
8 Miraculous Fixation of Molecular Nitrogen from the Atmosphere
147