Biological Nitrogen Fixation

Introduction

The Earth’s atmosphere is made up of 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide and trace quantities of other gasses. Nitrogen is also included in many biological components. Despite this, there is a very low concentration of nitrogen in the usable form. Nitrogen gas is essential, yet it is also a limited resource. It is required by all species, including humans, plants, and other animals, for their everyday biological processes. Now, the question is how the environment meets all these demands.

The process of converting atmospheric nitrogen into nitrogenous compounds by living organisms is called biological nitrogen fixation. Essentially, two main types of microorganisms are involved in biological nitrogen fixation: those that live in close symbiotic association with other plants, and those that are “free-living” or non-symbiotic.

Some examples of biological nitrogen fixation

In nature, certain microbes have the ability to fix atmospheric nitrogen. Some examples include the symbiotic Rhizobium and Frankia; and the free-living Azospirillum and Azotobacter.

Details of biological nitrogen fixation

Nitrogen occurs as an extremely inert di-nitrogen in over 80% of the Earth’s atmosphere, which most plants cannot use. Two nitrogen atoms are connected by a triple covalent bond in atmospheric dinitrogen (N2). Breaking this triple bond requires roughly 225 kcal of energy, which is tough to do. 

Now, let’s discuss the types of biological nitrogen fixation. 

Types of Biological Nitrogen Fixation:

Following are the types of nitrogen fixation: 

• Non- Symbiotic Biological Nitrogen Fixation
• Associative Biological Nitrogen Fixation
• Symbiotic Biological Nitrogen Fixation

Non- Symbiotic Biological Nitrogen Fixation

Non-symbiotic nitrogen fixation refers to biological nitrogen fixation by microorganisms that live outside a plant cell. Biological Nitrogen Fixation is a method of converting atmospheric nitrogen into nitrogenous compounds. A variety of free-living nitrogen-fixing microbes can be found in soil. Also, a variety of aerobic and anaerobic microorganisms, as well as blue-green algae, are among them.

The non-symbiotic nitrogen fixers can be classified as follows: 

1. Free-living aerobic Nitrogen-fixing bacteria: Photosynthetic: Chlorobium, Chromatium Non-Photosynthetic: Azotobacter, Azomonas, Dexia, Beijerinckia.
2. Free-living anaerobic Nitrogen-fixing bacteria: Photosynthetic: Rhodospirillum. Non-Photosynthetic: Clostridium.
3. Free-living chemosynthetic bacteria: Heterotrophic: Desulfovibrio.
4. Cyanobacteria or Blue-green algae: Heterocyst bearing: Nostoc, Anabaena, Rivularia, Calothrix. Non-Heterocyst bearing: Oscillatoria, Gloeocapsa, Lyngbya, Plectonema.
5. Free-living Fungi: Yeasts and Pullularia. The free-living symbiotic nitrogen fixers are primitive. Fixation is a reduction process that does not need respiration. Under low aeration, these organisms fix nitrogen more actively, as long as no hydrogen gas is generated.

Associative Symbiotic Nitrogen Fixation

Nitrogen is fixed by bacteria that live in proximity to the roots of cereals and grasses. Associative Symbiosis is the name given to this flexible mutualism. The bacteria live in the rhizosphere (the zone between soil and root) and occasionally invade the roots. Some fixed nitrogen is taken by the roots, which provides sustenance to the bacteria via the carbohydrates produced by the roots.

Some examples are :

1. Azospirillum brasilense in association with cereal roots. 
2. Beijerinckia in association with the roots of Sugarcane.
3. Azotobacter paspali in association with roots of tropical grass- Paspalum notatum.

Symbiotic Biological Nitrogen Fixation

Symbiotic nitrogen fixation is the exchange between plants and bacteria, in which one provides a niche and one provides fixed nitrogen. For example, Symbiotic biological nitrogen fixation can be categorised into the following groups:

 1. Nitrogen Fixation in leguminous plants: 

Symbiotic nitrogen fixers in several legume plants include the genus Rhizobium mainly. They established themselves inside specialised structures on the roots called root nodules. The bacteria fix nitrogen only when they are present inside the nodules. The association is regarded as symbiotic because the nodule bacteria is supplied by the host plant with the organic carbon (carbohydrates). The microorganisms then supply fixed Nitrogen to the host plant. 

 2. Nitrogen Fixation via nodule formation in the non-leguminous plants: 

Many plants belonging to families other than Leguminosae are known to produce root nodules. The important among them are primarily trees and shrubs. 

The key examples of non-leguminous plants that fix Nitrogen and produce root nodules are: 

1. Genus Frankia produces root nodules in association with Alnus, Myrica gale, Casuarina equisetifolia, etc. 
2. Rhizobium also has root nodules in the genus Parasponia. 
3. Leaf nodules are formed by bacteria Klebsiella in the genus Psychotria and by bacteria Burkholderia in genus Pavetta zimermanniana.
4. Nitrogen Fixation through Non-nodulation: In some plants, symbiotic nitrogen fixation occurs, but nodules are not formed. Such associations are Pseudo symbiotic (Pseudo Symbiosis). Some of the examples are: Anthoceros, associated with Nostoc; Azolla living in association with Anabaena.

Importance of Biological Nitrogen fixation

For appropriate growth and development, all plants, especially forage crops, require a lot of nitrogen (N). The process of incorporating nitrogen gas (N2) from the environment into the tissue of some plants is known as biological nitrogen fixation (BNF). Only a few plants can receive Nitrogen this way, thanks to soil microbes. Legumes (plants in the botanical family Fabaceae) are widely recognised among forage plants for their ability to acquire Nitrogen from air N2.

This process is critical in forage production since it implies that the much-needed Nitrogen may come from three places: the atmosphere via BNF, the soil and fertilizers. Forage growers who figure out how to get the most Nitrogen from the atmosphere via BNF may save money on fertilizer while maintaining soil fertility, high forage protein levels and high yields.

 A host plant is required for some forage crops to assimilate N2 from the air into their tissues (also known as the microsymbiont).

For example, in a symbiotic connection or Symbiosis, alfalfa and a microbe (also known as a microsymbiont) connected with the host plant work together. Two creatures develop a mutually beneficial interaction in a symbiotic relationship. The second organism in most forage crops is a bacterium that lives in the soil naturally. Because it is categorized as part of the bacterial genus Rhizobium, the bacterium most commonly associated with fodder crops is known as rhizobia.

These soil bacteria infect the roots of the plant, causing nodules to grow. Chemical processes occur in the nodules, which is known as the BNF process. Despite the fact that the process requires a lot of complicated biological processes, the following equation may be used to explain it in a reasonably basic manner :

      N2 + 8H2+ 16ATP ——> 2 NH3 + 2H2+ 16ADP + 16 Pi

As shown in the equation above, one molecule of nitrogen gas (N2) reacts with eight hydrogen ions (also known as protons) to generate two molecules of ammonia (2NH3) and two molecules of hydrogen gas (8H+) (2H2).

An enzyme called nitrogenase is responsible for this process. The energy necessary to carry out the BNF process is represented by the 16 molecules of ATP (ATP = Adenosine Triphosphate, an energy-storing chemical).16 ATP is a significant quantity of plant energy in biochemical terms. In terms of energy consumption, the BNF process is thus ‘expensive’ for the facility. Amino acids include nitrogen, which the plant may use to make proteins for its growth and development.

Conclusion

Nitrogen is a necessary ingredient for plant growth and development, but it is in short supply in its most common form, atmospheric nitrogen. Plants, on the other hand, rely on compounded or fixed nitrogen sources like ammonia and nitrate. Much of this nitrogen is supplied to cropping systems in the form of nitrogen fertilizers manufactured in factories. The use of these fertilisers has resulted in a slew of global environmental issues, including the establishment of coastal dead zones. Biological nitrogen fixation, on the other hand, provides plants with a natural source of nitrogen. It’s an important part of many aquatic and terrestrial ecosystems all around the world.