How are microbes classified according to carbon source?

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Classification by carbon source looks at where the organism gets its carbon for biosynthesis. Autotrophs fix carbon from inorganic carbon, typically carbon dioxide, using CO2 as their carbon source. Heterotrophs rely on organic carbon preformed in other organisms. Energy sources (light or chemical) can vary within these groups, but the carbon-source distinction remains: autotrophs use CO2, heterotrophs use organic carbon. That makes Autotrophs → CO2; Heterotrophs → Organic carbon the best way to state this concept.