In autotrophs, what is the initial carbon source?

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Autotrophs build their biomass from inorganic carbon, using carbon dioxide as the starting carbon source. They harvest energy from light (in photoautotrophs) or from inorganic chemical reactions (in chemoautotrophs) to drive the fixation of CO2 into organic molecules. Through carbon-fixation pathways like the Calvin cycle, CO2 is converted into sugars and other organic compounds that form the cell’s carbon skeleton. Organic carbon would require heterotrophy, and glucose is produced later in metabolism rather than being the initial carbon source. Methane can serve as an energy source for some organisms, but the carbon they ultimately incorporate into biomass still comes from CO2.