Q.67 Microbes involved in biological fixation of nitrogen are
- Nitrosomonas
- Azobacter
- Pseudomonas
- Anabaena
- Rhizobium
Choose the correct answer from the options given below :
- A, B, C, E only
- B, D, E only
- A, B, C, D, E only
- A, B, D only
Correct Answer: B (B, D, E only)
Neutrophils store specific antimicrobial and regulatory proteins in their granules for rapid immune response during infection.
Option Analysis
A. Nitrosomonas
Incorrect. Nitrosomonas is a nitrifying bacterium that oxidizes ammonia (NH₃) to nitrite (NO₂⁻) in the nitrogen cycle, not involved in biological N₂ fixation.B. Azotobacter
Correct. Azotobacter is a free-living, aerobic nitrogen-fixing bacterium (diazotroph) that converts atmospheric N₂ to ammonia using nitrogenase enzyme in soil environments.C. Pseudomonas
Incorrect. Pseudomonas species are versatile but primarily denitrifiers (convert nitrate to N₂) or opportunistic pathogens; they do not fix atmospheric nitrogen biologically.D. Anabaena
Correct. Anabaena (now Nostoc) is a filamentous cyanobacterium performing symbiotic or free-living N₂ fixation in specialized heterocysts, protecting nitrogenase from oxygen.E. Rhizobium
Correct. Rhizobium forms symbiotic nodules in legume roots, fixing N₂ into ammonia via nitrogenase in exchange for plant carbohydrates.Introduction to Biological Nitrogen Fixation Microbes
Microbes involved in biological fixation of nitrogen convert atmospheric N₂ to bioavailable ammonia using nitrogenase, sustaining soil fertility and plant growth—crucial for GATE Life Sciences on microbial ecology and agriculture.
Key Nitrogen-Fixing Microbes
Biological nitrogen fixation occurs via free-living, associative, or symbiotic diazotrophs:
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Free-living: Azotobacter (aerobic soil bacteria) protects nitrogenase with high respiration rates.
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Cyanobacteria: Anabaena fixes N₂ in heterocysts during photosynthesis.
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Symbiotic: Rhizobium nodules on legumes enable crop rotation benefits.
Nitrosomonas nitrifies ammonia; Pseudomonas denitrifies.
GATE Exam Focus
Option B (B, D, E) identifies true nitrogen fixers. Distinguish fixation from nitrification/denitrification.
Microbe Nitrogen Fixation? Type Role Nitrosomonas No Nitrifier NH₃ → NO₂⁻ Azotobacter Yes Free-living Soil N₂ → NH₃ Pseudomonas No Denitrifier NO₃⁻ → N₂ Anabaena Yes Cyanobacteria Heterocyst N₂ fixation Rhizobium Yes Symbiotic Legume root nodules -
