Which of the following is a nitrifying bacterium?

Step 1: Analyze the question We need the bacterium that carries out nitrification, the conversion of ammonia/ammonium into nitrites and then nitrates in the nitrogen cycle.

Step 2: The concept Nitrification happens in two stages:

1. Ammonia (NH₃) → Nitrite (NO₂⁻) by Nitrosomonas (and Nitrosococcus)
2. Nitrite (NO₂⁻) → Nitrate (NO₃⁻) by Nitrobacter

These bacteria are chemoautotrophs. They get energy from oxidizing nitrogen compounds.

Step 3: Check each option

• A. Rhizobium: a nitrogen-fixing bacterium living in root nodules of legumes (e.g., beans, groundnut). It converts atmospheric N₂ to ammonia, which is nitrogen fixation, not nitrification.
• B. Nictrobacter: this is a misspelling of Nitrobacter, which oxidizes nitrites to nitrates. It is a nitrifying bacterium.
• C. Azotobacter: a free-living nitrogen-fixing bacterium in the soil, so not nitrifying.
• D. Pseudomonas: a denitrifying bacterium, which converts nitrates back to nitrogen gas, so the opposite of nitrification.

Step 4: Answer Option B is the correct choice. The spelling "Nictrobacter" is clearly a typo for Nitrobacter, and it is the only nitrifying bacterium in the list.

Exam tip: Remember the groups by their roles:

• Fixers: Rhizobium, Azotobacter, Clostridium
• Nitrifiers: Nitrosomonas, Nitrobacter (the names start with "Nitro", for nitrate/nitrite)
• Denitrifiers: Pseudomonas, Thiobacillus denitrificans