Question

Which one is not a free living nitrogen fixer?
A. Rhodospirillum
B. Azoto Bacterial
C. Rhizobium
D. Bacillus

Answer 1

Hint: Nitrogen cycle refers to the regular circulation of nitrogen amongst the various components of the environment, ViZ., the living organisms, reservoir pool in the atmosphere and cycling pool in the lithosphere (soil). Nitrogen-fixation: The process of conversion of atmospheric nitrogen to ammonia is called nitrogen-fixation.

Complete answer:
Examples of free-living N,-fixing bacteria are Azotobacter and Beijerinckia (both aerobic). Bacillus Clostridium, Rhodospirillum are anaerobic, free-living,-fixing bacteria.
Examples of free-living nitrogen fixing cyanobacteria are Anabaena and Nostoc.
Examples of symbiotic nitrogen fixing bacteria are Rhizobium and Frankia.
Rhodospirillum :- it is anaerobic ( do not require oxygen ) free living nitrogen fixing bacteria. So this option is not correct.
Azotobacter :- it is an aerobic ( requires oxygen ) free living nitrogen fixing bacteria. So this is not correct.
Rhizobium :- it occurs in the roots of leguminous plants and fixes nitrogen by living in symbiotic association with them . It is not free living. So this option is correct.
Bacillus :- it is anaerobic free living bacteria that fix nitrogen . So this is not correct.

Our required answer is c that is Rhizobium.

Note: Nitrogen-fixation can occur in the following ways
Atmospheric nitrogen-fixation: In nature, lightning and ultraviolet radiations provide enough energy to allow nitrogen to combine with oxygen in order to form nitrogen oxides.
Industrial nitrogen-fixation : Industrial combustions, automobile exhausts and power generating stations are also sources of atmospheric nitrogen oxides.
Biological nitrogen-fixation: Reduction of inert atmospheric nitrogen to ammonia by living organisms Is called biological nitrogen-fixation.