Symbiotic And Non Symbiotic Nitrogen Fixation

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:56 PM IST

Definition Of Nitrogen Fixation

Nitrogen fixation is the process of converting atmospheric nitrogen, N₂, to a form that can be utilised by living organisms—for example, ammonia, NH₃. Nitrogen is an element critical to life. Nitrogen forms part of the composition of amino acids, proteins, DNA, and chlorophyll.

Without available nitrogen, plants can't grow, and the entire food web—animals and humans—would be disrupted. Nitrogen fixation may occur biologically with symbiotic bacteria like leguminous plants' Rhizobium or abiotically through natural occurrences like lighting and industrial processes, for example, the Haber-Bosch process.

Types Of Nitrogen Fixation

The types of nitrogen fixation are-

Biological Nitrogen Fixation

In the nitrogen cycle, the process of fixation is of prime importance and needs to be understood about other processes like nitrification, assimilation, ammonification and denitrification, which provide a source of nitrogen and its recycling in the ecosystem.

Enzyme nitrogenase

Catalyses reduction of atmospheric nitrogen (N₂) to ammonia (NH₃)
Anaerobic conditions to enable nitrogen fixation to occur

Symbiotic Nitrogen Fixation

The symbiotic Nitrogen fixation is described below-

The plants and nitrogen-fixing bacteria are in a mutual symbiotic relationship in which these.
The bacteria enter the roots of the plants, and special structures developed as a response termed nodules that perform the reaction of nitrogen fixation

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Types Of Symbiotic Nitrogen Fixation

The types of symbiotic nitrogen fixation are:

Legume-Rhizobium Symbiosis

Examples: Peas, beans, clover

Non-Legume Symbiosis

Examples: Alder trees with Frankia bacteria

Process Of Symbiotic Nitrogen Fixation

Infection: It is a process where the infection by bacteria requires attachment and penetration of the root hair.
Nodule formation: Plant cells increase and form nodules.
Nitrogen fixation: Atmospheric nitrogen is reduced to ammonia under the influence of bacteria in the nodules.

Importance Of Symbiotic Nitrogen Fixation

A better understanding and manipulation of nonsymbiotic nitrogen fixation can help in sustainable agriculture and hence global food security.
The process avails the important plant diet from the process so that high crop plants are achieved, ensuring sustainable agriculture.

Non-Symbiotic Nitrogen Fixation

The free-living nitrogen-fixing bacteria and the cyanobacteria do not enter into any specific association with plants. They do free-living nitrogen-fixing.

They convert atmospheric nitrogen into a biologically available form because of their metabolic activities.

Types Of Non-Symbiotic Nitrogen Fixation

The different types of non-symbiotic nitrogen fixation are:

Free-Living Bacteria
Examples: Azotobacter, Clostridium
Cyanobacteria
Examples: Anabaena, Nostoc

Process Of Non-Symbiotic Nitrogen Fixation

Activity of enzyme: The reaction needs to be carried out in the presence of the nitrogenase enzyme.
Nitrogenase: It reduces atmospheric nitrogen (N₂) to ammonia (NH₃).

Importance Of Non-Symbiotic Nitrogen Fixation

The non-symbiotic nitrogen fixation is important in:

Contribution To The Nitrogen Cycle

This appears to be a significant ecological function performed within natural ecosystems because it maintains the balance of nitrogen by converting atmospheric nitrogen from its inert form to a plant-available form.

Role In Natural Ecosystems

The process helps in the maintenance of primary productivity and thus the perpetuation of the ecosystems due to the constant replenishment of nitrogen in its reduced forms.

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Frequently Asked Questions (FAQs)

1. What is nitrogen fixation and why is it important?

Nitrogen fixation is a process whereby atmospheric nitrogen—N₂—is converted to a form taken up and used by plants, mainly in the form of ammonia or other nitrogenous compounds.

This is very important because most organisms cannot make use of atmospheric nitrogen directly; fixed nitrogen is involved in synthesizing proteins, nucleic acids, and other important molecules that support plant growth and productivity in ecosystems

2. What is nitrogen fixation and why is it important?

Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into a form that plants can use, such as ammonia (NH3). It's crucial because nitrogen is an essential element for plant growth and protein synthesis, but most plants cannot directly use the abundant nitrogen in the air. Nitrogen fixation makes this vital nutrient available to plants and other organisms in the ecosystem.

3. How does symbiotic nitrogen fixation work in legumes?

It is the symbiotic mutualism of the host leguminous plant and nitrogen-fixing bacterium, Rhizobium. These bacteria infect the root hair of the leguminous and fix nitrogen. They cause the formation of nodules in the roots of some leguminous plants.

Within these nodules, the bacteria are capable of atmospheric nitrogen fixation, which they then convert into ammonia in the presence of the nitrogenase enzyme. The plant provides carbohydrates from the plant itself and a poison-free environment to the bacteria, while they provide fixed nitrogen to the plants.

4. What are examples of non-symbiotic nitrogen-fixing bacteria?

Non-symbiotic nitrogen-fixing bacteria are as follows:

Azotobacter: Free-living nitrogen-fixing bacterium in neutral and alkaline soils.
Clostridium: An anaerobic bacterium that fixes nitrogen in the soil.
Cyanobacteria, Blue-Green Algae: Anabaena, Nostoc etc. fix nitrogen in aquatic environments.

5. What factors affect the efficiency of nitrogen fixation?

The efficiency of nitrogen fixation depends on several factors. It includes:

Oxygen Levels: High levels of oxygen in the atmosphere repress the action.
Soil pH: Optimum PH ranges since nitrogen-fixing bacteria grow and activity is optimal within some PH ranges.
Availability of Nutrients: Nitrogenase enzyme depends on the optimum availability of molybdenum iron and other nutrients.
Ecological Conditions: Temperature, Moisture, and light intensity may affect nitrogen-fixing organisms activity

6. How can nitrogen fixation benefit agriculture and the environment?

Nitrogen fixation serves agriculture by improving soil fertility naturally and, therefore, independently of synthetic nitrogenous fertilizer input. It will support sustainable agriculture by promoting plant growth and yield, mainly of leguminous crops, hence food security.

Biologically, nitrogen fixation maintains the nitrogen status in an ecosystem while at the same time helping to reduce nitrogen runoff and pollution generally for healthy and resilient ecosystems.

7. How do nitrogen-fixing organisms protect the nitrogenase enzyme from oxygen?

Nitrogen-fixing organisms use various strategies to protect nitrogenase from oxygen:

8. How do marine ecosystems benefit from nitrogen fixation?

Marine ecosystems benefit from nitrogen fixation in several ways:

9. How do nitrogen-fixing bacteria adapt to different oxygen concentrations in various environments?

Nitrogen-fixing bacteria adapt to different oxygen concentrations through various strategies:

10. What are the potential ecological impacts of introducing non-native nitrogen-fixing plants to an ecosystem?

Introducing non-native nitrogen-fixing plants can have several ecological impacts:

11. What are some examples of non-symbiotic nitrogen-fixing bacteria?

Examples of non-symbiotic nitrogen-fixing bacteria include Azotobacter, Clostridium, and Beijerinckia, which live freely in soil. Cyanobacteria (blue-green algae) like Nostoc and Anabaena are also capable of non-symbiotic nitrogen fixation in aquatic environments.

12. How does nitrogen fixation contribute to the nitrogen cycle?

Nitrogen fixation is a crucial part of the nitrogen cycle, converting atmospheric nitrogen into biologically available forms. This process adds new nitrogen to ecosystems, which can then be used by plants and other organisms. Fixed nitrogen moves through the food chain and is eventually returned to the atmosphere through processes like denitrification, completing the cycle.

13. How does symbiotic nitrogen fixation differ from non-symbiotic nitrogen fixation?

Symbiotic nitrogen fixation involves a mutually beneficial relationship between nitrogen-fixing bacteria and host plants, typically legumes. The bacteria live in root nodules and provide fixed nitrogen to the plant in exchange for carbohydrates. Non-symbiotic nitrogen fixation is carried out by free-living bacteria in the soil or water, without a direct association with plants.

14. How does the nitrogenase enzyme complex work in nitrogen fixation?

The nitrogenase enzyme complex is responsible for breaking the triple bond in atmospheric nitrogen (N2) and converting it to ammonia (NH3). This complex consists of two proteins: dinitrogenase reductase and dinitrogenase. The process requires a significant amount of energy in the form of ATP and reducing power, making it a metabolically expensive process.

15. What factors can limit nitrogen fixation in both symbiotic and non-symbiotic systems?

Factors that can limit nitrogen fixation include:

16. What are the main types of bacteria involved in symbiotic nitrogen fixation?

The primary bacteria involved in symbiotic nitrogen fixation are Rhizobium species, which form associations with legumes. Other examples include Frankia, which forms symbiotic relationships with non-leguminous plants like alder and casuarina.

17. Can you explain the role of root nodules in symbiotic nitrogen fixation?

Root nodules are specialized structures formed on the roots of legumes in response to infection by nitrogen-fixing bacteria. They provide a protected environment for the bacteria to live and carry out nitrogen fixation. The nodules also facilitate the exchange of nutrients between the plant and bacteria, with the plant providing carbohydrates and the bacteria supplying fixed nitrogen.

18. What is leghemoglobin and why is it important in nitrogen fixation?

Leghemoglobin is a protein found in root nodules that gives them a pink or red color. It plays a crucial role in nitrogen fixation by regulating oxygen levels within the nodule. Leghemoglobin binds to oxygen, maintaining a low oxygen environment necessary for the nitrogenase enzyme to function while still providing enough oxygen for the bacteria's respiration.

19. How do legumes benefit from symbiotic nitrogen fixation?

Legumes benefit from symbiotic nitrogen fixation by receiving a steady supply of fixed nitrogen from the bacteria in their root nodules. This allows them to grow in nitrogen-poor soils and reduces their dependence on external nitrogen sources. As a result, legumes often have higher protein content compared to non-leguminous plants.

20. How do plants recognize and form associations with nitrogen-fixing bacteria?

The process of recognition and association between plants and nitrogen-fixing bacteria involves a complex chemical dialogue. Legumes release flavonoids that attract compatible Rhizobium bacteria. In response, the bacteria produce Nod factors, which trigger root hair curling and the formation of infection threads. This leads to the development of root nodules where the bacteria can colonize and fix nitrogen.

21. What is the difference between autotrophic and heterotrophic nitrogen fixation?

Autotrophic nitrogen fixation is carried out by organisms that can produce their own food, such as cyanobacteria. They use energy from light (photosynthesis) to fix nitrogen. Heterotrophic nitrogen fixation is performed by organisms that require organic compounds for energy, like Rhizobium bacteria in root nodules, which rely on carbohydrates from their host plants.

22. What is the significance of nitrogen fixation in agriculture?

Nitrogen fixation is crucial in agriculture for several reasons:

23. What is the energetic cost of nitrogen fixation for bacteria and plants?

Nitrogen fixation is an energy-intensive process. For bacteria, breaking the triple bond in N2 requires a significant amount of ATP and reducing power. For plants in symbiotic relationships, they must provide carbohydrates to the bacteria, which can use up to 20-30% of the plant's photosynthetic output. This high energy cost is balanced by the benefit of accessing otherwise unavailable nitrogen.

24. How does climate change potentially impact nitrogen fixation?

Climate change can affect nitrogen fixation in several ways:

25. What are some industrial applications of nitrogen fixation?

While most industrial nitrogen fixation uses the Haber-Bosch process, there are some biological applications:

26. What is the role of lectins in symbiotic nitrogen fixation?

Lectins are proteins produced by legume plants that play a role in the recognition and attachment of compatible rhizobia to root hairs. They act as a sort of "glue" that helps the bacteria adhere to the plant cells, facilitating the initial stages of the symbiotic relationship and the formation of root nodules.

27. How does the presence of soil nitrogen affect symbiotic nitrogen fixation?

High levels of available soil nitrogen can inhibit symbiotic nitrogen fixation. When plants can easily obtain nitrogen from the soil, they may reduce or stop the energy-intensive process of maintaining symbiotic relationships with nitrogen-fixing bacteria. This is why adding nitrogen fertilizers to legume crops can sometimes be counterproductive.

28. How do some non-leguminous plants form nitrogen-fixing symbioses?

Some non-leguminous plants, such as alder and casuarina, form symbiotic relationships with nitrogen-fixing actinobacteria of the genus Frankia. These associations, called actinorhizal symbioses, result in the formation of root nodules similar to those in legumes. The process of recognition and nodule formation is different from legume-rhizobia symbioses but serves a similar function.

29. What is the difference between obligate and facultative nitrogen fixers?

Obligate nitrogen fixers are organisms that must fix nitrogen to survive. They cannot use other forms of nitrogen and rely entirely on their ability to fix atmospheric nitrogen. Examples include some species of cyanobacteria. Facultative nitrogen fixers can fix nitrogen but also use other nitrogen sources when available. Many symbiotic nitrogen fixers, like Rhizobium, are facultative, able to live freely in soil when not in symbiosis with plants.

30. How do free-living nitrogen fixers contribute to ecosystem nitrogen budgets?

Free-living nitrogen fixers, such as Azotobacter in soil or cyanobacteria in aquatic environments, contribute to ecosystem nitrogen budgets by:

31. What is associative nitrogen fixation and how does it differ from symbiotic fixation?

Associative nitrogen fixation occurs when nitrogen-fixing bacteria live in close proximity to plant roots without forming distinct nodules. These bacteria, such as Azospirillum, colonize the rhizosphere or even the intercellular spaces of the root cortex. While they fix nitrogen, the association is less structured and the nitrogen transfer to the plant is less direct compared to symbiotic fixation. The bacteria benefit from plant root exudates but don't form the highly specialized structures seen in symbiotic relationships.

32. What are the main differences between Rhizobium and Frankia in terms of their nitrogen-fixing symbioses?

The main differences between Rhizobium and Frankia symbioses are:

33. How does the process of nodule formation in legumes differ from nodule formation in actinorhizal plants?

The process of nodule formation differs between legumes and actinorhizal plants:

34. What role do plant hormones play in the establishment and maintenance of nitrogen-fixing symbioses?

Plant hormones play crucial roles in nitrogen-fixing symbioses:

35. How does the efficiency of nitrogen fixation compare between different types of nitrogen-fixing organisms?

The efficiency of nitrogen fixation varies among different organisms:

36. What are the key genes involved in nitrogen fixation, and how are they regulated?

The key genes involved in nitrogen fixation include: