Q.52 Which one of the following is a free-living photosynthetic nitrogen fixer?
(A) Frankia (B) Clostridium (C) Rhodospirillum (D) Rhizobium
Rhodospirillum is the free-living photosynthetic nitrogen fixer among the options.
Question Breakdown
This question from GATE Life Sciences assesses knowledge of nitrogen-fixing microorganisms, specifically those that are free-living (non-symbiotic), photosynthetic (use light energy), and capable of converting atmospheric N₂ to ammonia via nitrogenase.
Option Analysis
(A) Frankia
Frankia forms symbiotic actinorhizal associations with woody plants like alder, fixing nitrogen inside root nodules; it is not free-living or photosynthetic.
(B) Clostridium
Clostridium is a free-living, anaerobic, heterotrophic bacterium that fixes nitrogen fermentatively in soil/water, but lacks photosynthesis.
(C) Rhodospirillum
Rhodospirillum (purple non-sulfur bacteria) is free-living, photosynthetic (anaerobic/aerobic conditions using bacteriochlorophyll), and fixes nitrogen under microaerobic/light conditions.
(D) Rhizobium
Rhizobium is symbiotic, forming root nodules in legumes for nitrogen fixation; it is not free-living or photosynthetic.
Free-living photosynthetic nitrogen fixer bacteria like Rhodospirillum play key roles in soil fertility, fixing N₂ independently via photosynthesis.
Key Traits of Free-Living Photosynthetic Nitrogen Fixers
These bacteria use light for energy, protecting oxygen-sensitive nitrogenase by spatial/temporal separation from photosynthesis. Rhodospirillum thrives in aquatic/sediment environments, contributing to global nitrogen cycles.
Comparison Table
| Option | Free-Living? | Photosynthetic? | Nitrogen Fixation | Correct? |
|---|---|---|---|---|
| (A) Frankia | No (symbiotic) | No | Yes (symbiotic) | No |
| (B) Clostridium | Yes | No | Yes (anaerobic) | No |
| (C) Rhodospirillum | Yes | Yes | Yes | Yes |
| (D) Rhizobium | No (symbiotic) | No | Yes (symbiotic) | No |
GATE Exam Relevance
This PYQ emphasizes microbial ecology in Life Sciences (XL) papers. Focus on diazotroph classifications: symbiotic (Rhizobium, Frankia), free-living heterotrophic (Clostridium, Azotobacter), photosynthetic (Rhodospirillum, cyanobacteria).
