Mechanism of biological nitrogen fixation

The mechanism of biological nitrogen fixation is a process in which nitrogen is converted into usable form with the help of living organisms. This can be done by free living bacteria and symbiotic bacteria or symbiotic microorganisms. Nitrogen fixation occurs when molecular nitrogen is converted to ammonia. 

Examples of free living bacteria are Azotobacter, rhodospirillum, etc. An example of sympathetic bacteria is rhizobium (symbiotic with leguminous plants) etc. Certain cyanobacteria, such as Nostoc and Anabaena, help in nitrogen fixation in this category. 

Nostoc and Anabaena can fix nitrogen in a free state and a symbiotic state, whereas rhizobium helps in nitrogen fixation only in a symbiotic condition.

Mechanism of biological nitrogen fixation meaning

Rhizobium leguminosarum is a symbiotic species of rhizobium. In a symbiotic condition, it helps in nitrogen fixation. Rhizobium is an aerobic bacterium. It turns anaerobic when it is in a nitrogen-fixing state. Rhizobium has an enzyme called nitrogenase, which helps in nitrogen fixation.  

Nitrogenase is the molybdenum iron-containing protein (MoFeP). This one is active only in an anaerobic condition due to the presence of the enzyme, which helps in nitrogen fixation.

Mechanism of biological nitrogen fixation in legumes

The mechanism of biological nitrogen fixation in legumes takes place in the following steps:

• Nodule formation

In nodule formation, the infection of rhizobium bacteria to the plant occurs through the root hair. These nitrogenases are present in the soil, making an infection thread. In the formation of a nodule, the following steps take place:-

i) Formation of infection thread

Infection thread is a chain of these bacteria, and this chain contacts through the root hair and curls the root hair.

ii) Curling of root hair 

• After the infection, root hair curls and infection thread reach up to cortex cells in the root and the bacteria changes into two types. Some of them change into bacteroids.
• These bacteroides initiate cell division in cells of the cortex in the root, and when cortical cells divide, a mass of cells is formed and is called the nodule.
• Bacteroides initiate cell division, and nodule formation takes place. 
• The first step is the formation of an infection thread that is rhizobium. Infection thread enters the root hair, and it shows curling. This infection thread then reaches up to the cortex.
• Some of those bacteria change into bacteroides and initiate cell division of the cortical cell, forming nodules.
• Some cells of the cortex get modified into nitrogen-fixing cells. These are specialised cells having thick walls where nitrogen fixation can take place.
• These special cells are nitrogen fixation cells (tightly packed cells) which create an anaerobic condition for the nitrogenase enzyme to work. Nitrogen-fixing cells maintain an anaerobic condition.

2. Nitrogen fixation

• The conversion of molecular nitrogen into ammonia is known as nitrogen fixation.
• In the process of nitrogen fixation, the nitrogenase enzyme is essential.
• This nitrogenase enzyme has certain vital properties. It is a molybdenum iron-containing protein (MoFeP) made up of two subunits – iron-containing protein and iron and molybdenum-containing protein. These two subunits make the nitrogenase enzyme.

• We know that the nitrogenase enzyme can work only in an anaerobic condition, so there is a pigment present in a leguminous plant known as Leg Hb (leguminous haemoglobin).
• It acts as an oxygen scavenger. It removes all the oxygen and creates an anaerobic condition for nitrogenase to work.
• That is why rhizobium can work with leguminous plants, and this is a symbiotic association.
• Nitrogenase can work only in an anaerobic condition, and the roots of leguminous plants have leg haemoglobin which removes the oxygen and creates an anaerobic condition.

• Hydrogen donors are required for nitrogen fixation. Hydrogen donors can be many such as pyruvic acid, glucose, sucrose, etc.
• ATP as the source of energy is also required. One more carbon compound is needed to bind the ammonia molecules which are produced.

3. Nitrification 

• In nitrification, ammonia is converted into nitrites and nitrates.
• In this process, ammonia is first converted into an ammonium ion, and then this ammonium ion gets oxidised in the presence of some bacteria.
• These bacteria are found in soil. For example, nitrosomonas and nitrococcus.
• With the help of these bacteria, ammonium ion is oxidized into nitrite, and a water molecule is released.
• Now, nitrites are oxidised further with the help of some bacteria like nitrobacter and converted into nitrates again, and water molecules are produced. These nitrates are used by the plants.

4. Ammonification 

• Ammonification means the conversion of organic matter or any nitrogenous compounds into ammonia.
• Plants and animals decompose after their death, and this organic matter, with the help of bacteria, is converted into ammonia. This process is called ammonification.
• Similarly, animals excrete nitrogenous waste. These could be urea or uric acid. With industrial nitrogen fixation, we can also get urea as a fertiliser, and this urea is also converted into ammonia.
• Any organic matter on the nitrogen-containing compound is converted into ammonia, and this process is known as ammonification.

Conclusion

In the mechanism of biological nitrogen fixation in legumes, two nitrogen molecules are combined with two hydrogen molecules in the presence of 8 electrons, 8 protons, and 16 ATPs. Then, we get 2 ammonia molecules, 16 ADP (adenosine diphosphate) molecules, 16 inorganic phosphate, and two hydrogen molecules. 

So, two nitrogen, 8 electrons, 8 protons, and 16 ATP would give us 2 ammonia molecules and ADP, and inorganic phosphate would be released. This is called the mechanism of biological nitrogen fixation. This reaction takes place only in an anaerobic condition.