Question

Which is a free living nitrogen fixing bacterium?

• A. Pseudomonas
• B. Azotobacter
• C. Nitrosomonas
• D. Rhizobium

Answer 1

The correct option is B Azotobacter

Reduction of inert atmospheric nitrogen to ammonia by living organisms is called biological nitrogen-fixation. Only certain prokaryotic organisms (some bacteria and cyanobacteria) are capable of fixing $N_2$. Such microbes are called $N_2$-fixers. These $N_2$-fixing microbes contain an exclusive enzyme, Nitrogenase that is capable of reducing nitrogen.

Azotobacter is the free-living $N_2$-fixing bacteria. It is aerobic i.e., it requires oxygen for respiration.