Question

Which bacteria converts nitrite to nitrate?

• A. Nitrosomonas
• B. Nitrobacter
• C. Rhizobium
• D. Azotobacter

Hint:

Remember the alphabetic flow of the nitrification process:
Ammonia $\rightarrow$ Nitrite (by Nitrosomonas).
Nitrite $\rightarrow$ Nitrate (by Nitrobacter).
S comes before B in 'Nitroso' and 'Nitroba', matching the order of reactions.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the name of the bacterium that specifically oxidizes nitrite ($NO_2^-$) into nitrate ($NO_3^-$).
This is a standard question from the nitrogen cycle in mineral nutrition.

Step 2: Key Formula or Approach:
We examine the two distinct steps of the biological process called nitrification:
Step 1: Ammonia ($NH_3$) is oxidized to Nitrite ($NO_2^-$).
Step 2: Nitrite ($NO_2^-$) is oxidized to Nitrate ($NO_3^-$).

Step 3: Detailed Explanation:

• Nitrification is an aerobic process carried out by chemoautotrophic bacteria.

• The first step, converting ammonia to nitrite, is catalyzed by ammonia-oxidizing bacteria such as Nitrosomonas and Nitrosococcus:
\[ 2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O \]

• The second step, converting the toxic nitrite to the plant-absorbable nitrate, is catalyzed by nitrite-oxidizing bacteria such as Nitrobacter and Nitrococcus:
\[ 2NO_2^- + O_2 \rightarrow 2NO_3^- \]

• Let us evaluate the other options:

• Rhizobium is a symbiotic nitrogen-fixing bacterium associated with root nodules of legumes.

• Azotobacter is a free-living, aerobic nitrogen-fixing bacterium found in the soil.

• Since the question specifies the conversion of nitrite to nitrate, Nitrobacter is the correct organism.


Step 4: Final Answer:
The bacterium responsible for converting nitrite to nitrate is Nitrobacter.