What are nitrogen fixing bacteria definition types and process of nitrogen fixation
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?
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.
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 on Nitrogen Fixing Bacteria and Their Role in the Nitrogen Cycle
1. What are nitrogen fixing bacteria?
Nitrogen fixing bacteria are prokaryotic microorganisms that convert atmospheric nitrogen (N₂) into usable forms like ammonia (NH₃) through the process of nitrogen fixation.
These bacteria:
- Possess the enzyme nitrogenase to break the strong triple bond in N₂.
- Make nitrogen available to plants in the form of ammonia or related compounds.
- Play a crucial role in the nitrogen cycle and soil fertility.
2. How do nitrogen fixing bacteria fix nitrogen?
Nitrogen fixing bacteria fix nitrogen by using the enzyme nitrogenase to convert atmospheric N₂ into ammonia (NH₃) under anaerobic conditions.
The process involves:
- Absorption of atmospheric nitrogen.
- Reduction of N₂ to NH₃ using ATP and electrons.
- Incorporation of ammonia into organic molecules like amino acids.
3. What is the role of nitrogen fixing bacteria in the nitrogen cycle?
Nitrogen fixing bacteria initiate the nitrogen cycle by converting inert atmospheric nitrogen into biologically usable forms.
Their role includes:
- Producing ammonia from atmospheric N₂.
- Supplying nitrogen to plants for protein and nucleic acid synthesis.
- Maintaining soil fertility and ecosystem productivity.
4. What are examples of nitrogen fixing bacteria?
Common examples of nitrogen fixing bacteria include Rhizobium, Azotobacter, and Anabaena.
They can be classified as:
- Symbiotic bacteria: Rhizobium (in legume root nodules).
- Free-living bacteria: Azotobacter (in soil).
- Cyanobacteria: Anabaena and Nostoc (aquatic environments).
5. What is the difference between symbiotic and free-living nitrogen fixing bacteria?
The main difference is that symbiotic nitrogen fixing bacteria live in close association with plants, while free-living nitrogen fixing bacteria fix nitrogen independently in soil or water.
Key differences:
- Symbiotic: Example – Rhizobium in legume root nodules; receive nutrients from host plant.
- Free-living: Example – Azotobacter; survive independently in soil.
- Symbiotic forms are generally more efficient in nitrogen fixation.
6. Why are leguminous plants associated with nitrogen fixing bacteria?
Leguminous plants are associated with nitrogen fixing bacteria because they form a mutualistic symbiosis with Rhizobium in specialized root nodules.
In this association:
- Rhizobium fixes atmospheric nitrogen into ammonia.
- The plant supplies carbohydrates and a protected environment.
- The protein leghemoglobin regulates oxygen levels in nodules.
7. What is nitrogenase and why is it important?
Nitrogenase is the key enzyme complex that catalyzes the conversion of atmospheric nitrogen into ammonia during nitrogen fixation.
Its importance lies in:
- Breaking the strong triple bond of N₂.
- Functioning only under low-oxygen conditions.
- Enabling biological nitrogen fixation in bacteria and cyanobacteria.
8. Can nitrogen fixing bacteria survive in the presence of oxygen?
Nitrogen fixing bacteria generally require low-oxygen conditions because the enzyme nitrogenase is oxygen-sensitive.
Adaptations include:
- Formation of protective root nodules in legumes.
- Production of leghemoglobin to regulate oxygen.
- Specialized cells called heterocysts in cyanobacteria.
9. How do nitrogen fixing bacteria benefit agriculture?
Nitrogen fixing bacteria benefit agriculture by naturally increasing soil nitrogen levels and reducing the need for chemical fertilizers.
Their agricultural importance includes:
- Enhancing crop yield in leguminous plants.
- Improving soil fertility through biofertilizers.
- Supporting sustainable and eco-friendly farming practices.
10. Are nitrogen fixing bacteria autotrophic or heterotrophic?
Nitrogen fixing bacteria can be either autotrophic or heterotrophic depending on the species.
Examples include:
- Heterotrophic bacteria: Rhizobium and Azotobacter obtain carbon from organic compounds.
- Autotrophic cyanobacteria: Anabaena perform photosynthesis and fix nitrogen.
