Which of the following microorganisms can fix nitrogen symbiotically?
• [(A).] Anabaena
• [(B).] Azotobacter
• [(C).] Frankia
• [(D).] Ulothrix
• [(E).] Bradyrhizobium
Choose the correct answer from the options given below:
Which of the following microorganisms can fix nitrogen symbiotically?
• [(A).] Anabaena
• [(B).] Azotobacter
• [(C).] Frankia
• [(D).] Ulothrix
• [(E).] Bradyrhizobium
Choose the correct answer from the options given below:
Show Hint
- Symbiotic: Rhizobium, Bradyrhizobium, Frankia, Anabaena azollae.
- Free-living (Asymbiotic): Azotobacter (aerobic), Clostridium (anaerobic).
- Associative: Azospirillum.
- (A), (B) and (E) only.
- (A), (C) and (E) only.
- (A), (D) and (E) only.
- (A), (C), (D) and (E) only.
The Correct Option is B
Solution and Explanation
Biological Nitrogen Fixation (BNF) is the biochemical process where diazotrophic microorganisms convert inert atmospheric nitrogen gas (N$_2$) into ammonia (NH$_3$).
This process is classified as either free-living (asymbiotic) or symbiotic, depending on whether the microbe establishes a physical, mutually beneficial relationship with a host plant.
Step 3: Detailed Explanation:
Let us evaluate each microorganism to determine its mode of nitrogen fixation:
- Anabaena (A): This is a genus of filamentous cyanobacteria.
It forms a well-known symbiotic association with the water fern Azolla (Anabaena azollae), residing in the fern's leaf cavities and fixing nitrogen to support the host's growth. Thus, (A) is correct.
- Frankia (C): This is a genus of actinomycetes (filamentous, Gram-positive bacteria).
It forms symbiotic nitrogen-fixing root nodules on non-leguminous angiosperms, collectively known as actinorhizal plants (such as Alnus, Casuarina, and Myrica). Thus, (C) is correct.
- Bradyrhizobium (E): This is a genus of slow-growing, Gram-negative soil bacteria.
It forms classic symbiotic relationships with leguminous crops, particularly soybean (Glycine max), producing root nodules to fix nitrogen. Thus, (E) is correct.
Let us analyze the incorrect options:
- Azotobacter (B): This is a genus of free-living, aerobic, heterotrophic bacteria that fixes nitrogen independently in the soil. It does not form symbiotic structures with plants.
- Ulothrix (D): This is a genus of non-branching filamentous green algae. It is eukaryotic and lacks the genetic machinery (nif genes) required to fix nitrogen.
Therefore, the symbiotic nitrogen-fixers among the choices are (A), (C), and (E).
Step 4: Final Answer:
The correct combination is (A), (C) and (E) only, matching Option (B).
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