Q.82 Which one of the following pairs of bacterial species fixes atmospheric Nitrogen?
(A) Clostridia and Rhizobia
(B) Clostridia and Lactobacillus
(C) Rhizobia and Enterococcus
(D) Actinomycetes and Mycoplasma
The correct answer is (A) Clostridia and Rhizobia. Both are well-established nitrogen-fixing bacteria: Clostridia as free-living anaerobes and Rhizobia as symbiotic partners with legumes.
Question Context
This tests knowledge of biological nitrogen fixation, where bacteria convert atmospheric N₂ to ammonia via nitrogenase. GATE Life Sciences often features such microbial metabolism questions.
Option Analysis
(A) Clostridia and Rhizobia
Clostridia (e.g., Clostridium pasteurianum) are free-living, anaerobic nitrogen fixers found in soil. Rhizobia form root nodules in legumes, fixing N₂ symbiotically. Both possess nitrogenase; this is the only correct pair.
(B) Clostridia and Lactobacillus
Clostridia fix nitrogen, but Lactobacillus ferments sugars to lactic acid and lacks nitrogenase. It cannot fix atmospheric N₂.
(C) Rhizobia and Enterococcus
Rhizobia fix nitrogen symbiotically, but Enterococcus is a gut commensal involved in fermentation, not nitrogen fixation.
(D) Actinomycetes and Mycoplasma
Actinomycetes like Frankia fix nitrogen in actinorhizal plants (not all), but Mycoplasma lack cell walls and nitrogenase, functioning as parasites.
Introduction
Understanding bacterial species fixes atmospheric nitrogen is crucial for pairs of bacterial species in GATE Life Sciences. Clostridia and Rhizobia are the key nitrogen fixing bacteria that convert N₂ to ammonia, unlike Lactobacillus, Enterococcus, or Mycoplasma.
Nitrogen Fixation Process
Nitrogenase reduces N₂ to NH₃ in microaerophilic/anaerobic conditions. Free-living bacteria (Clostridia, Azotobacter) fix in soil; symbiotic ones (Rhizobia, Frankia) form nodules providing plant-fixed carbon.
Correct Pair Breakdown
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Clostridia: Anaerobic, soil-dwelling, free-living N-fixers.
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Rhizobia: Gram-negative, legume symbionts forming pink nodules with leghemoglobin.
Comparison Table
| Pair | Nitrogen Fixation Capability | Type |
|---|---|---|
| Clostridia + Rhizobia | Both fix N₂ | Free-living + Symbiotic |
| Clostridia + Lactobacillus | Only Clostridia | Fermenter |
| Rhizobia + Enterococcus | Only Rhizobia | Gut commensal |
| Actinomycetes + Mycoplasma | Partial (Frankia only) | Wall-less parasite |
GATE Exam Tips
Focus on classic fixers: Rhizobium (symbiotic), Clostridium/Azotobacter (free-living), Cyanobacteria, Frankia. Eliminate non-nitrogenase bacteria like Lactobacillus/Mycoplasma.
