Rizo

 Rhizobium: nitrogen  fixing bacteria  

What Is Rhizobium?

Rhizobium is the bacteria that live in symbiotic association with the root nodules of the leguminous plants. Fixation of nitrogen cannot be done independently. That is why rhizobium requires a plant host. Rhizobium is a vital source of nitrogen to agricultural soils including those in arid regions. They convert dinitrogen into ammonia. Ammonia, being toxic in nature.

Classification of Rhizobium Bacteria Rhizobium can be classified on the basis of the types of the plant they are associated with and also the rate of growth. Few species of Rhizobium bacteria include: 

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

What Is Nitrogen Fixation?

Nitrogen fixation is the essential biological process and the initial stage of the nitrogen cycle. In this process, the free nitrogen available in the atmosphere is converted into ammonia (another form of nitrogen) by certain bacterial species like Rhizobium, Azotobacter, etc. and the complete process is carried on by natural phenomena.

Role of Rhizobium In Nitrogen Fixing

Rhizobium plural form rhizobia are prokaryotes whose main function involves the conversion of stable nitrogen gas in the atmosphere to a biologically useful form. Nitrogenase is an enzyme complex that reduces dinitrogen to ammonia.

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).