Question

Name any one symbiotic nitrogen-fixing bacteria. How does it help in nitrogen fixation?

Answer 1

Hint: Biological nitrogen fixation was discovered by Hermann Hellriegel and Martinus Beijerinck. In the environment, Biological nitrogen fixation (BNF) occurs that converts atmospheric nitrogen into ammonia. This occurs in the presence of nitrogenase enzyme The nitrogen fixation process is converting atmospheric nitrogen into a usable form which can be consumed by the plants. These are carried out by nitrogen fixation bacteria.

Complete answer:
The biological nitrogen is the nitrogen from the air for the nutrition of the leguminous plants by nitrogen fixation bacteria which is present in the roots of these plants. Legumes and rhizobium are a fabulous example of symbiosis in nature. Diazotrophs are the microorganisms in which nitrogen fixation takes place and they occur naturally in the soil. For example – bacteria such as Azotobacter and archaea.

- Legumes, like other plants, cannot fix nitrogen on their own.
- Rhizobium is a bacterium that possesses the necessary nitrogenase enzymes to fix nitrogen but has difficulty functioning on its own in soil.
- The legume provides a happy home for rhizobium but goes a step further. Since atmospheric oxygen interferes with the function of nitrogenase, the plant provides a substance, leghemoglobin, to "scrub" the oxygen from the air.
- Moreover, the immediate product from nitrogen fixation is ammonia, which would be toxic to the plant.
- So rhizobium returns the favor by immediately converting the ammonia to amino acids that the plant can use to construct proteins.

Note: Rhizobium is a free-living bacterium in the soil. Whenever it recognizes the roots of leguminous plants it makes the infection to root hair and is established in the root by forming a nodule. Inside the nodule bacteria converts to Bacteroides and produces the enzyme nitrogenase. This is the enzyme that converts atmospheric nitrogen to ammonia. But this enzyme is very sensitive to oxygen. So to remove oxygen from the nodule, the host plant produces leghemoglobin, which acts as an oxygen scavenger.