Question

Identify the bacteria which can fix atmospheric nitrogen and are free-living in the soil.
(A) Azotobacter
(B) Rhizobium
(C) Azospirillum
(D) Anabaena
Choose the correct answer from the options given below:

• A. (A) and (C) only
• B. (B) and (D) only
• C. (B), (C) and (D) only
• D. (A), (B) and (C) only

Hint:

Nitrogen Fixers:
$\bullet$ Free-living bacteria: Azotobacter, Azospirillum, Beijerinckia.
$\bullet$ Symbiotic bacteria: Rhizobium (legumes), Frankia (non-legumes like Alnus).
$\bullet$ Cyanobacteria: Anabaena, Nostoc, Oscillatoria.

Answer 1

The Correct Option is A

Concept:
Nitrogen-fixing microorganisms convert molecular atmospheric dinitrogen ($N_2$) into usable ammonia ($NH_3$). They are classified as symbiotic or free-living organisms.

Step 1: Classifying the Nitrogen-Fixing Microorganisms:
- Azotobacter: An aerobic, free-living soil bacterium capable of fixing atmospheric nitrogen independently without forming symbiotic plant associations. (Statement A is free-living bacterium).
- Azospirillum: A free-living/associative nitrogen-fixing bacterium found living freely in the rhizosphere and soil, enriching the soil nitrogen content. (Statement C is free-living bacterium).
- Rhizobium: A symbiotic nitrogen-fixing bacterium that forms root nodules specifically with leguminous plants; it cannot fix nitrogen when living freely in soil. (Statement B is symbiotic).
- Anabaena: A cyanobacterium (blue-green alga) that fixes nitrogen symbiotically (e.g., in Azolla and Cycas coralloid roots) or in aquatic environments, but is classified as a photosynthetic cyanobacterium rather than a typical free-living soil bacterium.

Step 2: Selecting the Correct Combination:
The free-living nitrogen-fixing bacteria in the soil explicitly recognized together in textbook literature are Azotobacter and Azospirillum.
Hence, statements (A) and (C) are the correct choices.

Step 3: Final Answer:
The matching choice is Option (A).