Nitrogen-fixing-bacteria
Introduction to nitrogen-fixing-bacteria:
Nitrogen forms one of the important constituent in the atmosphere and is also highly required in living organisms. Nitrogen is the key component of biomolecules like amino acids, proteins, lipids and nucleic acids and also forms a key component of nitrogenous wastes in animals.
Such important constituent is cannot be directly utilized by plants and other living forms without the help of some specialized nitrogen fixing bacteria. These are the special bacteria which possess some specializing genes called nitrogen fixing genes and form symbiotic association with plants. Upon symbiotic association these bacteria fix the nitrogen and supply for plants in form of nitrate and ammonia and in return get some food.
Free Living nitrogen fixing bacteria
a) Obligate anaerobes -- Clostridium pasteurianum
b) Facultative anaerobes -- Klebsiella,
c) Photosynthetic bacteria -- Rhodobacter
d) Most cyanobacteria
e) Obligate aerobes such as Azotobacter
f) Some methanogens
Symbiotic nitrogen fixing bacteria
As nitrogen fixation takes place under anerobic conditions the symbiotic bacteria needs to be protected from inhabitation of oxygen. Hence the symbiotic bacteria with help of some nodular genes develop nodules on roots of plant and start the process of nitrogen fixation. Examples are Rhizobium and Bradyrhizobium.
Most common bacteria called rhizobioum though found freely in soil but can fix nitrogen once it gains symbiotic association with root nodules in which strict microaerophilic environment is possible.
Organisms like Frankia have symbiotic association with Actinorhizal plants similar to leguminous plants.
Cyanobacteria like anabena, known to be in symbiotic association with fern plant called Cycas.
It is proven that either free living or symbiotic nitrogen fixing bacteria are able to fix nitrogen as part of their metabolic pathway.
- Nitrogen Fixation (Conversion of free nitrogen to nitrite or ammonium ion)
- Nitrification (Conversion of ammonia to nitrite)
- Assimilation (Intake of ammonia and nitrite into biological tissues)
- Ammonification (Conversion of organic nitrogen compounds into ammonia)
- Denitrification (Conversion of nitrite ion to nitrogen)
