How Do Nitrogen Fixing Bacteria Help Plants Thrive?
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 Explained
1. What is nitrogen fixation, and why is this process so important for plants and other living organisms?
Nitrogen fixation is the biological process of converting atmospheric nitrogen (N₂), which is largely unusable by most organisms, into ammonia (NH₃) or other nitrogen compounds. This process is crucial because nitrogen is a fundamental component of essential organic molecules like amino acids, proteins, nucleic acids (DNA and RNA), and chlorophyll. Since plants cannot use the N₂ gas directly from the atmosphere due to its strong triple covalent bond, they rely on nitrogen-fixing bacteria to make this vital nutrient available to them, thereby supporting the entire food web.
2. What are the main types of nitrogen-fixing bacteria? Please provide examples for each.
Nitrogen-fixing bacteria can be broadly classified into two main types based on their lifestyle:
Symbiotic Nitrogen-Fixing Bacteria: These bacteria live in a mutually beneficial relationship with a host plant. They form nodules on the roots of the plant and directly supply it with fixed nitrogen. The most well-known example is Rhizobium, which associates with leguminous plants like peas, beans, and clover. Another example is Frankia, which forms nodules in non-leguminous plants like Alnus.
Free-Living (Asymbiotic) Nitrogen-Fixing Bacteria: These bacteria live independently in the soil and fix nitrogen without a host. They enrich the soil with nitrogenous compounds upon their death and decomposition. Examples include Azotobacter (aerobic), Clostridium and Beijerinckia (anaerobic), and cyanobacteria like Anabaena and Nostoc.
3. What is the fundamental difference between nitrogen-fixing bacteria and nitrifying bacteria?
The fundamental difference lies in their roles within the nitrogen cycle:
Nitrogen-fixing bacteria perform nitrogen fixation. They take inert atmospheric nitrogen (N₂) and convert it into ammonia (NH₃), making nitrogen available to the ecosystem for the first time. Examples include Rhizobium and Azotobacter.
Nitrifying bacteria perform nitrification. They do not fix atmospheric nitrogen. Instead, they act on the ammonia already present in the soil, converting it first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). This nitrate form is the primary form of nitrogen absorbed by most plants. Examples include Nitrosomonas (converts ammonia to nitrite) and Nitrobacter (converts nitrite to nitrate).
4. How do symbiotic bacteria like Rhizobium establish a connection with leguminous plants to form root nodules?
The formation of root nodules is a complex, coordinated interaction between Rhizobium and the host legume. The process involves several steps:
The bacteria multiply around the plant's roots and attach to the root hairs.
In response, the root hairs curl, trapping the bacteria.
The bacteria then invade the root hair by forming an infection thread, which grows inwards and carries the bacteria to the cortex of the root.
Once in the cortex, the bacteria are released and stimulate the host cells to divide rapidly, leading to the formation of a mature root nodule.
Within these nodules, the bacteria differentiate into bacteroids and begin fixing atmospheric nitrogen.
5. What are the specific roles of the enzyme nitrogenase and the pigment leghaemoglobin in the process of nitrogen fixation?
Both are critical for biological nitrogen fixation, but they perform distinct functions:
Nitrogenase: This is the key enzyme complex responsible for the actual conversion of atmospheric nitrogen (N₂) into ammonia (NH₃). The reaction is extremely energy-intensive, requiring a significant amount of ATP. Crucially, nitrogenase is highly sensitive to oxygen and is irreversibly damaged by its presence.
Leghaemoglobin: This pinkish, iron-containing pigment acts as an oxygen scavenger. It is found in the root nodules of leguminous plants. Its role is to bind with oxygen, thereby creating the necessary anaerobic (oxygen-free) conditions inside the nodule to protect the nitrogenase enzyme from being inactivated, allowing it to function effectively.
6. Why is the nitrogenase enzyme sensitive to oxygen, and what adaptations do different nitrogen-fixing bacteria have to overcome this issue?
The nitrogenase enzyme is sensitive to oxygen because its metallic cofactors can be irreversibly oxidised, rendering it inactive. Since many nitrogen-fixing organisms live in aerobic environments, they have evolved several remarkable adaptations:
Symbiotic Adaptation: In legume root nodules, the host plant produces leghaemoglobin to create an anaerobic environment specifically around the nitrogen-fixing bacteroids.
Respiratory Protection: Free-living aerobic bacteria like Azotobacter have an extremely high rate of cellular respiration. They consume oxygen at their cell surface so rapidly that it prevents oxygen from reaching the nitrogenase enzyme inside the cell.
Compartmentalisation: Some cyanobacteria, like Anabaena, compartmentalise the process. They perform nitrogen fixation in specialised, thick-walled cells called heterocysts, which lack the oxygen-producing photosynthetic machinery (Photosystem II), thus keeping the internal environment anaerobic.
7. Besides leguminous plants, can other types of plants also form symbiotic relationships for nitrogen fixation?
Yes, nitrogen-fixing symbiotic relationships are not exclusive to legumes. Several non-leguminous plants also form such associations. The most notable example is the relationship between the actinomycete bacterium Frankia and the roots of certain woody plants like Alnus (alder) and Casuarina. Similar to Rhizobium, Frankia induces the formation of root nodules and fixes atmospheric nitrogen, contributing significantly to soil fertility in forest ecosystems.
