Question

Given below are two statements:
Statement (I): Nitrobacteria are obligate autotrophic aerobes
Statement (II): Nitrate will not be produced from NH$_4^+$ in the absence of oxygen

• 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:

Nitrifying bacteria like Nitrobacter require oxygen to convert ammonium to nitrate via nitrite.

Answer 1

The Correct Option is A

Nitrobacteria are obligate autotrophic aerobes that oxidize nitrite to nitrate. This nitrification step requires oxygen, so without oxygen, nitrate production from ammonium is halted.

Answer 2

Biochemical pathway approach:
Nitrification occurs in two enzymatic steps, both requiring molecular oxygen as the terminal electron acceptor, ammonia-oxidisers convert \( \text{NH}_4^+ \) to nitrite, and nitrite-oxidisers such as Nitrobacter, an obligate chemoautotroph, convert nitrite to \( \text{NO}_3^- \) using \( \text{O}_2 \) in their electron transport chain. Since Nitrobacter derives energy solely from this oxygen-dependent oxidation and cannot function anaerobically, both its obligate autotrophic aerobe classification and the dependency of nitrate formation on oxygen availability hold true, confirming that Statement I and Statement II are both correct.