Question

Which is not a free-living asymbiotic nitrogen fixation microorganism?

• A. Rhodospirillum
• B. Rhizobium
• C. Nostoc
• D. Anabaena

Hint:

Rhizobium is "Social." It only starts working (fixing nitrogen) once it moves into its "Home" (the Legume nodule).

Answer 1

The Correct Option is B

Concept: Nitrogen-fixing microbes are broadly divided into free-living (asymbiotic) and symbiotic organisms. Symbiotic organisms require a host plant to perform efficient fixation.

Step 1: Analyzing Asymbiotic (Free-living) Microbes.

• {Rhodospirillum (1) is a free-living anaerobic photosynthetic bacterium.
• {Nostoc (3) and {Anabaena (4) are cyanobacteria that can fix nitrogen independently in the soil or water (though they can also form symbioses, they are fundamentally capable of free-living fixation).

Step 2: Analyzing Rhizobium.
{Rhizobium (2) is the classic example of a symbiotic bacterium. While it can survive in the soil as a free-living cell, it generally cannot fix nitrogen in its free-living state. It must first infect the root of a legume and differentiate into a bacteroid within a root nodule to activate its nitrogen-fixing machinery.

Step 3: Conclusion.
Because Rhizobium's nitrogen fixation is strictly tied to its symbiotic relationship with legumes, it is not considered an asymbiotic fixer.