Answer: Rhizobium.
- A Nitrosomonas
- B Rhizobium
- C Azotobacter
- D Nitrobacter
Correct answer: B. Rhizobium
Explanation: Rhizobium is a symbiotic nitrogen-fixing bacterium that lives in root nodules of legumes (pea, bean, soybean). It has a mutualistic relationship with the plant - it gets sugar, plant gets fixed nitrogen.
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.