Carbon source of nitrifying bacteria: From where do Nitrosomonas and Nitrobacter obtain their carbon for growth?
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AOrganic compounds (heterotrophy)
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BCarbon dioxide (autotrophy)
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CElemental carbon (graphite/soot)
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DNo external carbon is required
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EMethane exclusively
Answer
Correct Answer: Carbon dioxide (autotrophy)
Explanation
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:
Identify energy source → inorganic oxidation.Identify carbon source → CO2 fixation (autotrophy).Select the correct option → carbon dioxide.Verification / Alternative check:Environmental microbiology texts classify both genera as obligate chemolithoautotrophs in standard culture conditions.
Why Other Options Are Wrong:
Organic compounds: would indicate heterotrophy, not typical for classical nitrifiers.Elemental carbon: biologically unavailable in this context.No carbon required: impossible for biomass formation.Methane exclusively: describes methanotrophs, not nitrifiers.Common Pitfalls:Confusing energy source (inorganic) with carbon source (CO2); both are distinct aspects of metabolism.
Final Answer:Carbon dioxide (autotrophy).