Symbiotic Nitrogen Fixation 

(Rhizobium–legume interaction, nodule formation, leghaemoglobin function)

Symbiotic nitrogen fixation is a biological process in which certain microorganisms convert atmospheric nitrogen (N₂) into ammonia (NH₃) while living in a mutually beneficial association (symbiosis) with plants.

• Atmospheric nitrogen (N₂) is inert and cannot be directly used by plants.
• Symbiotic bacteria (like Rhizobium) live inside plant roots.

• These bacteria possess the enzyme nitrogenase, which converts:

N₂→NH₃

• The ammonia is then assimilated into amino acids and other nitrogenous compounds.

It occurs in

• Mainly in leguminous plants (e.g., peas, beans, clover).
• Specialized structures called root nodules are formed.
• Inside nodules, bacteria differentiate into bacteroids (active nitrogen-fixing form)

Microbial Physiology and Metabolism