The bacterium in a mutualistic association with legumes converts

Step 1: Analyze the question The question asks about the bacterium living in a mutualistic association with legumes (beans, cowpea, groundnut, etc.). This is Rhizobium, which lives in root nodules.

Step 2: Explain the concept This is nitrogen fixation:

• Rhizobium lives in the root nodules of legumes.
• The plant gives the bacteria sugars (food) and a protected home.
• The bacteria give the plant usable nitrogen in the form of ammonia (which becomes ammonium compounds and then amino acids).
• This is why legumes enrich the soil, and why farmers use crop rotation with beans or cowpea.

Step 3: Check each option

• A. Nitrates to ammonia: This is nitrate reduction, not the work of Rhizobium. Incorrect.
• B. Ammonium compounds to nitrites: This is nitrification (stage 1), done by Nitrosomonas, which is free-living. Incorrect.
• C. Nitrogen gas to ammonia: This is nitrogen fixation, done by Rhizobium. Correct.
• D. Nitrites to nitrates: This is nitrification (stage 2), done by Nitrobacter. Incorrect.

Step 4: Answer The bacterium converts atmospheric nitrogen gas (N₂) to ammonia (NH₃).

Tip: Remember "Rhizobium = Roots = Fixes N₂." Also match the bacteria to their jobs:

• Rhizobium → N₂ to NH₃ (fixation)
• Nitrosomonas → NH₃ to nitrite
• Nitrobacter → nitrite to nitrate
• Pseudomonas (denitrifying) → nitrate to N₂