Question

Which of the following is not an anaerobic $N_2$ fixing organism?

Answer 1

Among the given microorganisms, Azotobacter is an aerobic, free-living microbe that converts the atmospheric nitrogen to ammonia which is utilized by the plants.

Clostridium pasteurianum is the first known anaerobic free-living bacterium that fixing nitrogen.

A purple non sulphur anoxygenic phototrophic bacterium, Rhodopseudomonas palustris grew fixing $N_2$ using aromatic compounds as the sole carbon source/electron donor.

Rhodospirillum rubrum is also a nitrogen fixing bacterium, i.e., it can express and regulate nitrogenase, a protein complex that can catalyse the conversion of atmospheric dinitrogen into ammonia.

Hence, the correct answer is option “A” – Azotobacter.