Carbon source of nitrifying bacteria: From where do Nitrosomonas and Nitrobacter obtain their carbon for growth?

Introduction / Context:
Nitrifying bacteria, such as Nitrosomonas (ammonia oxidizers) and Nitrobacter (nitrite oxidizers), derive energy from inorganic oxidation but must still assimilate carbon. This question tests whether you know their trophic strategy.

Given Data / Assumptions:

• Nitrification is a chemolithotrophic process.
• Energy comes from oxidation of NH3 to NO2− and NO2− to NO3−.
• Carbon assimilation can be autotrophic (CO2) or heterotrophic (organic C).

Concept / Approach:
Nitrifiers are chemolithoautotrophs: they fix CO2 (often via the Calvin–Benson cycle or related pathways) using energy obtained from inorganic electron donors. They do not rely on organic carbon as their main source.

Step-by-Step Solution:

Verification / Alternative check:
Environmental microbiology texts classify both genera as obligate chemolithoautotrophs in standard culture conditions.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing energy source (inorganic) with carbon source (CO2); both are distinct aspects of metabolism.

Final Answer:
Carbon dioxide (autotrophy).