Question

Given below are two statements:

Statement (I): Nitrobacteria are obligate autotrophic aerobes
Statement (II): Nitrate will not be produced from $\text{NH}_4^+$ in the absence of oxygen

In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct
• B. Both Statement (I) and Statement (II) are incorrect
• C. Statement (I) is correct but Statement (II) is incorrect
• D. Statement (I) is incorrect but Statement (II) is correct

Hint:

Because nitrification requires oxygen, waterlogged or flooded soils lack nitrate production, which limits nitrogen availability unless crops can directly absorb ammonium.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nitrification is a biological process that converts ammonium to nitrate, driven by specific groups of specialized soil bacteria.

Step 2: Detailed Explanation:
Statement (I) is correct because nitrifying bacteria (such as Nitrosomonas and Nitrobacter) are obligate autotrophs (they obtain carbon from \(\text{CO}_2\) and energy from inorganic nitrogen oxidation) and obligate aerobes (they require free gaseous \(\text{O}_2\) to act as an electron acceptor).
Statement (II) is correct because the biological oxidation of ammonium to nitrate is strictly aerobic:
\[ \text{NH}_4^+ + 5\text{O}_2 \rightarrow \text{NO}_2^- + \text{H}_2\text{O} + 2\text{H}^+ \]
\[ \text{NO}_2^- + 0.5\text{O}_2 \rightarrow \text{NO}_3^- \]
Because molecular oxygen (\(\text{O}_2\)) is required at both stages, no nitrate can be produced from ammonium under anaerobic (oxygen-depleted) conditions.
Therefore, both statements are true.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct.