Biological Nitrogen Fixation (BNF) is the process where atmospheric nitrogen gas (N2), unusable by most organisms, is converted into ammonia (NH3) by specialized microorganisms. This ammonia is then assimilated by plants, crucial for protein and nucleic acid synthesis. The enzyme nitrogenase, which requires energy (ATP) and anaerobic conditions, catalyzes this conversion.

Mechanism of Symbiotic Nitrogen Fixation (Examples: Legumes with Rhizobia):

1. Nodule Formation:

   • Leguminous plants (e.g., peas, soybeans) release flavonoids from roots, attracting Rhizobia bacteria.

   • Rhizobia infect root hairs, forming an "infection thread" into cortical cells.

   • Infected cortical cells proliferate, forming specialized root nodules. Rhizobia differentiate into bacteroids within these.

2. Nitrogen Fixation:

   • Inside bacteroids, nitrogenase converts N2 to NH3.

   • The plant produces leghaemoglobin (a red pigment) to maintain a low-oxygen, anaerobic environment in the nodule, essential for nitrogenase activity.

   • The plant supplies carbohydrates for the high ATP demand of fixation.

3. Ammonia Assimilation: Fixed NH3 is rapidly converted into ammonium and then amino acids by the plant, which are transported throughout.

This mutualistic relationship provides carbohydrates and protection to bacteria, while the plant receives usable nitrogen, reducing reliance on external fertilizers.