Carbon sources for biosynthesis Which organisms typically obtain their cell carbon primarily from carbon dioxide (CO₂) rather than from organic substrates?

Introduction / Context:
Microbes differ in both energy and carbon acquisition. While heterotrophs require organic carbon, autotrophs fix CO₂ into biomass. The question asks which organisms usually derive their biomass carbon from CO₂.

Given Data / Assumptions:

• Fermenters and respirers of glucose use organic carbon both for energy and biomass.
• Chemolithotrophs oxidize inorganic electron donors and often fix CO₂.
• Ammonia-oxidizing bacteria are classic chemolithoautotrophs.

Concept / Approach:
Chemolithoautotrophs (e.g., nitrifiers) oxidize reduced inorganic compounds for energy and use that energy to fix CO₂ as their carbon source. Ammonia-oxidizing bacteria such as Nitrosomonas oxidize NH₃ to nitrite and assimilate CO₂, fitting the definition precisely. In contrast, organisms using glucose—whether fermenting or respiring—are heterotrophs for carbon, not CO₂ fixers.

Step-by-Step Solution:
Identify which group is chemolithoautotrophic.Recognize ammonia oxidizers as archetypal CO₂ fixers among bacteria.Select the option naming ammonia-oxidizing (chemolithotrophic) bacteria.

Verification / Alternative check:
Nitrifiers are central to nitrogen cycling and primary production in some ecosystems due to their CO₂ fixation capability.

Why Other Options Are Wrong:

• Glucose users (aerobic/anaerobic; fermenters): use organic carbon; do not rely on CO₂ as sole carbon source.
• Lactic acid bacteria: also heterotrophs, using sugars from milk.

Common Pitfalls:
Mixing up energy source (inorganic vs organic) with carbon source (CO₂ vs organic). Chemolithotrophy commonly pairs with autotrophy.

Final Answer:
Ammonia-oxidizing bacteria (chemolithotrophic bacteria).