Autotrophy

Autotrophy is a mode of metabolism in which organisms synthesize organic compounds from inorganic carbon sources, usually carbon dioxide, using energy obtained from light or from inorganic chemical reactions. Autotrophs are contrasted with heterotrophs, which rely on preformed organic carbon.

Photoautotrophs use light energy to fix carbon and build biomass. This group includes plants, algae, and cyanobacteria, which carry out oxygenic photosynthesis that releases molecular oxygen. Some photosynthetic bacteria, such as green sulfur and purple sulfur bacteria, are also photoautotrophs but perform anoxygenic photosynthesis and do not produce O2.

Chemosynthetic, or chemoautotrophic, organisms obtain energy from the oxidation of inorganic molecules such as hydrogen, reduced sulfur compounds, ammonia, or ferrous iron. They fix CO2 into organic matter using this energy. Examples include nitrifying bacteria, sulfur-oxidizing bacteria, and many archaea such as methanogens and halophiles. Autotrophic pathways vary among groups and include several carbon fixation cycles.

Common carbon fixation pathways include the Calvin cycle, which is dominant in plants and cyanobacteria, and several anaerobic or thermophilic pathways such as the reverse TCA cycle (found in some bacteria and archaea), the reductive acetyl-CoA (Wood–Ljungdahl) pathway, and the 3-hydroxypropionate cycle.

Ecologically, autotrophs form the base of most food webs by converting inorganic carbon into organic matter that fuels heterotrophs. They also drive global biogeochemical cycles and have a long evolutionary history, with oxygenic photosynthesis significantly shaping Earth's atmosphere.