Autotrophs are organisms that primarily use which carbon source?

• A. CO2
• B. Glucose
• C. Organic carbon from other organisms
• D. Methane

Answer: (A)

Autotrophs obtain their carbon from inorganic sources by fixing CO2 into organic matter. They synthesize biomass starting from carbon dioxide (or bicarbonate in aquatic environments) using energy from light in photosynthesis or from the oxidation of inorganic substances in chemosynthesis. This is why carbon dioxide is the carbon source for autotrophs. Glucose represents preformed organic carbon that heterotrophs would use directly as fuel and building blocks, rather than fixing inorganic carbon themselves. Organic carbon from other organisms likewise fits the heterotroph pattern. Methane can serve as an energy source for some microbes, but the carbon autotrophs incorporate into their biomass still comes from CO2.