Microbial nutrition – carbon sources and classifications All organisms require at least small amounts of carbon dioxide (CO₂). However, some microorganisms can use CO₂ as their sole source of carbon for biosynthesis. Such organisms are termed:

Introduction / Context:
Microorganisms are classified by how they obtain energy and carbon. Understanding these terms is fundamental for environmental microbiology, biogeochemical cycles, and bioprocess design. The question asks for the specific term used for organisms that fix carbon dioxide (CO₂) as their sole carbon source to build cellular material.

Given Data / Assumptions:

• All living cells require a carbon source to synthesize biomolecules.
• Some microbes assimilate inorganic CO₂; others require preformed organic carbon such as sugars or organic acids.
• The classification focuses on the carbon source (not necessarily the energy source).

Concept / Approach:
Organisms that use CO₂ as their exclusive or principal carbon source are called autotrophs. They reduce CO₂ to organic carbon via pathways such as the Calvin–Benson–Bassham cycle, the reverse TCA cycle, or the Wood–Ljungdahl pathway. Autotrophs can be phototrophic (using light as energy) or chemolithotrophic (using energy from oxidation of inorganic compounds). The key feature is CO₂ fixation, independent of whether energy comes from light or chemicals.

Step-by-Step Solution:
Identify the property: ability to use CO₂ as sole carbon source.Map property to term: organisms with this trait are autotrophs.Differentiate from energy terms: phototroph and chemotroph describe energy, not carbon source.Conclude the correct classification is 'Autotrophs'.

Verification / Alternative check:
Examples include cyanobacteria (photoautotrophs) and nitrifiers like Nitrosomonas (chemolithoautotrophs). These organisms fix CO₂ and support global primary production in diverse habitats.

Why Other Options Are Wrong:

• Phototrophs: use light for energy; may be auto- or heterotrophic for carbon.
• Chemotrophs: use chemical compounds for energy; carbon source may be CO₂ or organic.
• Photo-organotrophs: not a standard primary category; suggests light energy with organic carbon (photoheterotrophy) rather than CO₂ fixation.
• Mixotrophs: combine strategies and do not necessarily rely solely on CO₂.

Common Pitfalls:
Confusing energy source (photo vs chemo) with carbon source (auto vs hetero). Always parse both axes when classifying metabolism.

Final Answer:
Autotrophs.