Which of following organisms cannot fix nitrogen ? A. Azotobacter B. OscillatoriaC. Anabaena D. VolvoxE. NostocChoose the correct answer from the options givenbelow:

Answer

Biological Nitrogen Fixation by Microorganisms

Nitrogen fixation is the biological process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) or related nitrogenous compounds in the soil, making nitrogen available to plants. This process is mainly facilitated by certain bacteria and cyanobacteria.

1. Symbiotic Nitrogen Fixation

• Rhizobium bacteria form a symbiotic relationship with the roots of leguminous plants.
• They induce the formation of root nodules, where they reside and fix nitrogen.
• These bacteria convert atmospheric nitrogen into organic compounds that the plant can use as nutrients.
• Benefits:
  • The plant receives usable nitrogen.
  • The bacteria receive carbohydrates and a protective environment from the plant.

2. Free-Living Nitrogen-Fixing Bacteria

• Some bacteria, like Azospirillum and Azotobacter, fix nitrogen while living freely in the soil.
• They enrich the nitrogen content of the soil, promoting plant growth naturally.

3. Cyanobacteria (Blue-Green Algae)

• Cyanobacteria are autotrophic microbes found in both aquatic and terrestrial habitats.
• Many cyanobacteria can fix atmospheric nitrogen.
• Examples include:
  • Anabaena
  • Nostoc
  • Oscillatoria

Summary

Microorganisms involved in nitrogen fixation, whether symbiotic or free-living, are crucial for:

• Improving soil fertility
• Reducing the need for chemical fertilizers
• Promoting sustainable agriculture