Question

Which feature of some species of blue-green algae helps promote them as bio-fertilizers?
1. They convert atmospheric methane into ammonia which the crop plants can absorb readily.
2. They induce the crop plants to produce the enzymes which help convert atmospheric nitrogen to nitrates.
3. They have the mechanism to convert atmospheric nitrogen into a form that the crop plants can absorb readily.
4. They induce the roots of the crop plants to absorb the soil nitrates in larger quantities.

Answer 1

Correct Answer - Option 3 : They have the mechanism to convert atmospheric nitrogen into a form that the crop plants can absorb readily.

The correct answer is They have the mechanism to convert atmospheric nitrogen into a form that the crop plants can absorb readily.

• Blue-green algae (Cyanobacteria) as biofertilizers:
  • Both free-living and symbiotic species of cyanobacteria are known to fix nitrogen from the air eg: Anabaena, Nostoc, Aulosira, Tolypothrix, etc.
  • Aulosira - It is the most active nitrogen-fixing blue-green alga.
  • Nostoc - It is known for providing nitrogen to rice crops.
  • Anabaena azollae - It forms a symbiotic relationship with Azolla (water fern). When such Azolla is grown in paddy fields, some nitrogen fixed by Anabaena gets dissolved in the water of the paddy and becomes available to rice plants. 
• Mycorrhiza as biofertilizers:
  • It is of two types - Endomycorrhiza and Ectomycorrhiza.

• Nitrogen-fixation:
  • The process by which atmospheric nitrogen is converted into nitrates, that can be utilized by the plants, is termed Nitrogen-fixation.
• Bio-fertilizers:
  • Fertilizers of biological are of two types - manures and biofertilizers.
  • The organisms which bring about soil nutrient enrichment are known as biofertilizers.
  • These are of following three types:
    • Bacteria
    • Cyanobacteria (blue-green algae)
    • Mycorrhiza (fungi)
  • Bacteria as biofertilizers:
    • Free-living Nitrogen-fixing bacteria:
      • eg: Azotobacter, Beijerinckia, Clostridium
    • Rhizosphere Nitrogen-fixing bacteria:
      • eg: Azospirillum
    • Symbiotic Nitrogen-fixing bacteria:
      • eg: Rhizobium (root nodules of leguminous plants), Xanthomonas (leaf nodules of Ardisia), Aerorhizobium caulinodans (stem nodules of Sesbania).