Role of Rhizobium in N2 fixation: Rhizobia are one of several group of bacteria capable of ‘fixing’ nitrogen, i.e. converting dinitrogen gas into ammonia and then into organic molecules such as amino acids. Because of this ability, nitrogen fixing bacteria are significant conduits between an extremely large pool of nitrogen in the atmosphere and living things who otherwise could only obtain nitrogen by recycling it from existing pools of organic nitrogen (e.g. amino acids, ammonia, nitrate and nitrite).

Unlike many nitrogen fixing bacteria that can fix nitrogen when ‘free-living‘ (i.e. when not living inside a host plant) Rhizobia can only fix nitrogen when associated with a plant that provides it with carbohydrates. The carbohydrates provide energy for a process that requires substantial inputs of energy (both ATP and the reducing power of NADH). Rhizobia only associate with legumes, members of the pea family. (But not all legumes associate with Rhizobia and some that have nitrogen fixing associates may have bacteria other than Rhizobia).

• Rhizobium leguminosarum
• Rhizobium alamii
• Rhizobium lantis
• Rhizobium japonicum
• Rhizobium trifolii
• Rhizobium phaseolii
• Rhizobium smilacinae

Role of Rhizobium in N2 fixation

• It is also known as biological nitrogen fixation where atmospheric or molecular nitrogen is converted into ammonia by an enzyme named nitrogenase.
• It converts free nitrogen into nitrogenous salts and helps in making it available for the absorption of plants.
• The biochemical reaction involved in nitrogen fixation is: N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂
• The reduction of N2 into NH3 requires 6 protons and 6 electrons where 12 molecules of ATP are also involved.
• A huge amount of energy is consumed during the nitrogen fixation and the nitrogenase enzymes are irreversibly inactivated by oxygen. 
• Rhizobium infects the roots of leguminous plants. They are usually found in the soil and produce nodules after infecting the roots of the leguminous plants. As a result, nitrogen gas is fixed from the atmosphere. 
• Specific strains of Rhizobium are required to make the nodules functional in order to carry out the process. This increases the yield of the crops. Legume inoculation has been an agricultural practice for several years and has constantly improved over time.

Types of Biological Nitrogen fixation

• Free living nitrogen fixing bacteria
• Symbiotic nitrogen-fixing bacteria
• Free living nitrogen fixing cyanobacteria
• Free living nitrogen fixing cyanobacteria

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By Team Learning Mantras