Question

Which is not a free living nitrogen fixing bacteria?
A. Azotobacter
B. Rhizobium
C. Bacillus
D. Rhodospirillum

Answer 1

Hint: Nitrogen fixing bacteria are microorganisms that convert atmospheric nitrogen into inorganic compounds that can be used by plants. Nitrogen fixing bacteria are of two types: Free living and symbiotic bacteria.

Complete answer:
As we already mentioned, nitrogen fixing bacteria convert atmospheric nitrogen into inorganic compounds for plants. Nitrogen fixing bacteria are of two types- the first type is free living (non-symbiotic) bacteria, which includes cyanobacteria, Nostoc and Anabaena, Rhodospirillum, Azotobacter, Clostridium etc. Some free-living bacteria fix enough nitrogen to grow without added nitrogen, but due to limited supply of energy, nitrogen fixation is limited in nature.

The second type of nitrogen fixing bacteria is mutualistic (symbiotic) bacteria. They include Azospirillum species, associated with cereal grasses, Rhizobium, associated with leguminous plants. The symbiotic bacteria are supplied with high-energy products from photosynthesis. So, the rate of N2 fixation is more in legumes. It mainly depends on plant species and cultivar, growing season, and soil fertility.

We have four options: Azotobacter, Rhizobium, Bacillus, and Rhodospirillum.
Among them, Azotobacter are free-living, aerobic bacteria.
Rhizobium is symbiotic nitrogen-fixing aerobic bacteria but fixes nitrogen in anaerobic conditions. Rhizobia are diazotrophic bacteria, it is established inside the root nodules of legumes (Fabaceae) and then fixes nitrogen for the plant. Bacillus is aerobic, ubiquitous (both free living and mutualistic) nitrogen fixing bacteria.
Rhodospirillum is a free-living nitrogen-fixing anaerobic bacteria. So, Rhizobium is not free living bacteria.

Hence, the correct answer is option (B).

Note: Rhizobium is symbiotic bacteria because it requires a plant host to express its genes for nitrogen fixation, they cannot express the gene for nitrogen fixation and can't fix nitrogen independently. So, they infect the roots of legumes and produce nodules in the legumes.