The bacterium which lives symbiotically in the root nodules of leguminous plants and fix atmospheric nitrogen is:

The bacterium which lives symbiotically in the root nodules of leguminous plants and fix atmospheric nitrogen is: Correct Answer Rhizobium

Concept:

The nitrogen cycle has four steps-

• Nitrogen Fixation- Conversion of molecular nitrogen into the inorganic nitrogenous compound.
• Ammonification- Conversion of the dead organic nitrogenous compound into ammonia.
• Nitrification- Oxidation of ammonia into nitrates.
• Denitrification- Nitrites or nitrates converts back into molecular free nitrogen.

Key PointsRhizobium

• It is a symbiotic nitrogen-fixing bacteria. 
• Rhizobium is found in the nodules of legumes and is responsible for nitrogen fixation.
• This was discovered in 1886 by the German physiologists Heleriegal and Wilfarth.  

• Vibrio is a gram-negative, comma-shaped bacteria.

Thus, the bacterium which lives symbiotically in the root nodules of leguminous plants and fixes atmospheric nitrogen is Rhizobium.

Additional Information 

• Nitrifying bacteria are chemolithotrophic organisms that include species of the genera e.g. Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrospina, Nitrospira, and Nitrococcus.
• Nitrifying bacteria convert the most reduced form of soil nitrogen, ammonia, into its most oxidized form, nitrate.
• In itself, this is important for soil ecosystem function, in controlling losses of soil nitrogen through leaching and denitrification of nitrate.
• Nitrifying bacteria: Nitrosomonas, Nitrosococcus, Nitrobacter (nitrifying bacteria convert ammonium compounds into nitrates)
• Ammonifying bacteria: Bacillus mycoides (ammonifying bacteria convert amino acids into ammonium compounds)
• Non-symbiotic Nitrogen-fixing bacteria:  Clostridium (nitrogen fixation bacteria convert atmospheric nitrogen to nitrates)
• Symbiotic Nitrogen-fixing bacteria: Rhizobium