Which of the following is a nitrifying bacterium?

Step 1: Analyze the question The question asks which bacterium is a nitrifying bacterium. Nitrifying bacteria are involved in nitrification, the conversion of ammonia into nitrites and then into nitrates in the soil.

Step 2: The concept Nitrification happens in two stages:

• Ammonia (NH₃) / ammonium → nitrite (NO₂⁻), done by Nitrosomonas (and Nitrosococcus)
• Nitrite (NO₂⁻) → nitrate (NO₃⁻), done by Nitrobacter

Plants absorb the nitrates, so this process is important for soil fertility.

Step 3: Check each option

• A. Azotobacter: a free-living nitrogen-fixing bacterium. It converts atmospheric N₂ into ammonia compounds. This is nitrogen fixation, not nitrification.
• B. Clostridium: an anaerobic bacterium that can fix nitrogen in the soil. It is not a nitrifier.
• C. Nitrobacter: oxidizes nitrites to nitrates. This is a nitrifying bacterium.
• D. Rhizobium: lives in root nodules of legumes (beans, cowpea) and fixes nitrogen symbiotically. It is not a nitrifier.
• E. Staphylococcus: a bacterium that causes boils and skin infections. It has no role in the nitrogen cycle.

Step 4: Answer The correct option is **C.

Tip: Look at the name. Nitro- bacter and Nitroso- monas are the "nitrifiers" (they make nitrites and nitrates). For nitrogen fixers, remember "A, R, C": Azotobacter, Rhizobium, Clostridium. These fix nitrogen, while the nitrifiers oxidize it into usable nitrates.