The carbon cycle in microorganisms is part of a larger cycling of carbon that occurs on the global scale. The actions of microorganisms help extract carbon from non-living sources and make the carbon available to living organisms (including themselves).

The cycling of carbon by microorganisms, including a variety of bacteria and fungi, occurs in aquatic habitats. Even relatively oxygen-free zones such as in the deep mud of lakes, ponds and other water bodies can be regions where the anaerobic conversion of carbon takes place.

Much of the carbon that enters the carbon cycle of microorganisms is carbon dioxide. This form of carbon exists as a gas in the atmosphere and can be dissolved in water. The atmospheric carbon dioxide can be converted to organic material in the process of photosynthesis. Photosynthetic algae are important microorganisms in this regard. As well, chemoautotrophs, primarily...