Nitrogen-Fixing Bacteria

Introduction to Nitrogen-Fixing Bacteria

Apart from carbon, hydrogen, and oxygen, nitrogen is the most abundant element in living organisms. Nitrogen is the basis of amino acids, proteins, hormones, chlorophylls, and many vitamins. Plants compete with microbes for the limited nitrogen which is available in the soil. Thus Nitrogen is a limiting nutrient for living beings. The nitrogen molecule remains as two nitrogen atoms joined by a very strong triple covalent bond. The process of conversion of atmospheric inert nitrogen gas to fixed nitrogen (inorganic compounds usable by plants, that is ammonia) is termed Nitrogen Fixation. Only certain prokaryotic bacterias are capable of fixing nitrogen. The reduction of free atmospheric nitrogen to ammonia by living organisms is called biological nitrogen fixation.


What are Nitrogen-Fixing Bacteria?

Nitrogen-fixing bacteria are microorganisms ( not visible in naked eyes) that are mainly prokaryotic. These Nitrogen-fixing microbes could be free-living or symbiotic. The enzyme, nitrogenase, which is capable of nitrogen reduction is present in Nitrogen-fixing bacterias.


Which Do Bacteria Bring About the Process of Nitrogen Fixation?

Only certain prokaryotic bacterias are capable of fixing nitrogen. These Nitrogen-fixing microbes could be free-living or symbiotic. The free-living (non-symbiotic) bacterias are the free-living cyanobacteria (or blue-green algae) Anabaena and Nostoc, free-living Nitrogen-fixing aerobic bacterias are Azotobacter, Beijernicika, and Clostridium, while Rhodospirillum is anaerobic and bacillus is free-living. Several types of symbiotic nitrogen-fixing associations are known. The most important among them is the legume-bacteria relationship. 

Species of Rhizobium have such a relationship with the roots of several legumes ( various members of the pea family) such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. The microbe Frankia also produces Nitrogen-fixing nodules on the roots of non-leguminous, dicotyledonous plants (actinorhizal plants); and certain Azospirillum species, associated with cereal grasses.


What is the Role of Nitrogen-Fixing Bacteria?

Very few living organisms can utilize the nitrogen in form of N2, available abundantly in the atmosphere. Only certain prokaryotic bacterias are capable of fixing nitrogen. Nitrogen is the basis of amino acids, proteins, hormones, chlorophylls, and many vitamins. More than 90% of all nitrogen fixation is done by Nitrogen-fixing bacterias. That is how they play an important role in the Nitrogen Cycle. The enzyme, nitrogenase, which is capable of nitrogen reduction is present in Nitrogen-fixing bacterias.


Nitrogen-Fixing Bacteria Examples

The free-living (non-symbiotic) bacterias are the free-living cyanobacteria (or blue-green algae) Anabaena and Nostoc, free-living Nitrogen-fixing aerobic bacterias are Azotobacter, Beijernicika, and Clostridium, while Rhodospirillum is anaerobic and bacillus is free-living.

Examples of symbiotic nitrogen-fixing bacteria are Rhizobium, associated with leguminous plants; Frankia, associated with actinorhizal plants; and Azospirilum, associated with cereal grasses.


Symbiotic Nitrogen-Fixing Bacteria

Several types of symbiotic nitrogen-fixing associations are known. The most important among them is the legume-bacteria relationship. Species of Rhizobium have such a relationship with the roots of several legumes (various members of the pea family) such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. The most common association on roots is nodules. These nodules are small outgrowths on trees. 

The center portion is red due to the presence of leghemoglobin. The microbe Frankia also produces Nitrogen-fixing nodules on the roots of actinorhizal plants; and certain Azospirillum species, associated with cereal grasses. Both Rhizobium and Frankia are free-living in soil, but only as symbionts, can fix atmospheric Nitrogen. Within the nodules, the bacteria convert free nitrogen to ammonia which is used by the host plant.


Did You Know?

  1. Species of Rhizobium has such a relationship with the roots of several legumes (various members of the pea family) such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. The most important association on roots is nodules. These modules are small outgrowths on trees. The centre portion is red due to the presence of leghemoglobin.

  2. Anabaena and Nostoc are cyanobacteria (blue-green algae) are free-living nitrogen-fixers, free-living Nitrogen-fixing aerobic bacterias are Azotobacter, Beijernicika, and Clostridium, while Rhodospirillum is anaerobic and bacillus is free-living.

Conclusion

Nitrogen is very essential for the support of life. Plants cannot use atmospheric Nitrogen directly. But some of the plants in association with Nitrogen-fixing bacteria, especially roots of leguminous plants, can fix atmospheric Nitrogen in biologically usable form. Ammonia produced by Nitrogen fixation is incorporated into amino acids as an amino group.

FAQs (Frequently Asked Questions)

1. What are Nodules and Describe their Role in Nitrogen Fixation By Symbiotic Bacteria:-

Answer. Nodules are small outgrowths on trees. The center portion is red due to the presence of leghemoglobin.

Very few living organisms can utilize the nitrogen in form of N2, available abundantly in the atmosphere. More than 90% of all nitrogen fixation is done by Nitrogen-fixing bacterias. Species of Rhizobium have such a relationship with the roots of several legumes (various members of the pea family) such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. Besides Rhizobium, the microbe Frankia also produces Nitrogen-fixing nodules on the roots of non-leguminous, dicotyledonous plants (actinorhizal plants). Thus nodules help in Nitrogen fixation.

2. Why Does Atmospheric Nitrogen Need to be Fixed?

Answer. Nitrogen is very essential for the support of life. Nitrogen is the basis of amino acids, proteins, hormones, chlorophylls, and many vitamins. Plants cannot use atmospheric Nitrogen directly. Plants compete with microbes for the limited nitrogen which is available in the soil. Thus Nitrogen is a limiting nutrient for living beings. The nitrogen molecule remains as two nitrogen atoms joined by a very strong triple covalent bond. Very few living organisms can utilize the nitrogen in form of N2, available abundantly in the atmosphere. Only certain prokaryotic bacteria are capable of fixing nitrogen. More than 90% of all nitrogen fixation is done by Nitrogen-fixing bacteria. That is why Nitrogen-fixing bacteria have a vital role in the nitrogen cycle.