How is carbon incorporated into microbial biomass?

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Carbon becomes part of microbial biomass through two main lifestyles: autotrophy and heterotrophy. Autotrophs fix inorganic carbon, typically CO2, into organic molecules that form the building blocks of the cell. They use energy from light (phototrophy) or from inorganic chemical reactions (chemoautotrophy) to drive this carbon fixation. Heterotrophs, on the other hand, obtain carbon by assimilating preformed organic compounds from the environment and converting those into cellular components. In both paths, carbon is incorporated into the macromolecules that make up biomass.

The other options mix energy-yielding processes or energy sources with carbon handling, but they don’t define how carbon is put into biomass. Photosynthesis and respiration describe energy and carbon flux in specific contexts, while fermentation and respiration are mainly about energy balance. Chemolithotrophy and phototrophy describe energy sources rather than the carbon source strategy.