Answer: Anabaena.
- A Rhizobium
- B Azotobacter
- C Anabaena
- D Nitrosomonas
Correct answer: C. Anabaena
Explanation: Anabaena (and Nostoc) are cyanobacteria (blue-green algae) that fix atmospheric nitrogen. Anabaena azollae grows in symbiosis with Azolla (water fern) in rice paddies, naturally fertilizing the water.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.