Nitrifiers and electron donors Bacteria of the genus <em>Nitrosomonas</em> obtain the electrons used for energy conservation by oxidizing which compound?

Introduction / Context:
Nitrifying bacteria drive the first step of nitrification in the nitrogen cycle. Nitrosomonas species are chemolithoautotrophs that oxidize reduced nitrogen compounds to harvest energy while fixing CO₂ for biomass. Knowing their electron donor is crucial in wastewater treatment and environmental microbiology.

Given Data / Assumptions:

• Nitrosomonas perform ammonia oxidation.
• Electron donors define chemolithotrophic niches.
• Alternative donors in the options represent other metabolisms.

Concept / Approach:
Nitrosomonas oxidize ammonia (NH₃/NH₄⁺) to nitrite (NO₂⁻), transferring electrons through their respiratory chain to generate proton motive force and ATP. They are not phototrophs and do not use typical organic donors like succinate; those are common in chemoorganoheterotrophs. H₂S is used by sulfur-oxidizers, not by classic ammonia oxidizers.

Step-by-Step Solution:
Identify organismal role: ammonia-oxidizing bacterium.Match role to electron donor: ammonia provides electrons upon oxidation to nitrite.Select 'Ammonia' as the correct electron source.

Verification / Alternative check:
Nitrification engineering models (e.g., activated sludge) explicitly allocate ammonia as the substrate for Nitrosomonas, confirming this choice.

Why Other Options Are Wrong:

• H₂S: substrate for sulfur oxidizers like Acidithiobacillus.
• Succinate: organic donor for heterotrophs.
• Light: used by phototrophs.
• H₂: donor for hydrogen-oxidizing bacteria, not canonical Nitrosomonas.

Common Pitfalls:
Confusing different chemolithotroph groups (nitrifiers vs sulfur oxidizers vs hydrogen oxidizers).

Final Answer:
Ammonia.