Answer: Symbiotic fixers.
- A Free-living fixers
- B Symbiotic fixers
- C Denitrifying microbes
- D Photosynthetic algae
Correct answer: B. Symbiotic fixers
Explanation: They fix nitrogen only in partnership with a host plant's roots, so they are symbiotic (not free-living) nitrogen fixers.
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.