An autotroph is an organism which is capable of obtaining all the carbon it requires from carbon dioxide, the main usable inorganic form of carbon on Earth. Autotrophs can get their energy either from light (phototrophs) or from chemical reactions (chemotrophs); the latter are almost always lithotrophs.
The most familiar autotrophs are the oxygen-releasing photosynthesizers: plants, algae, and cyanobacteria, but many other prokaryotes are also autotrophic, being not only phototrophic like plants, but also chemotrophic.
There are a variety of carbon-fixation methods used for obtaining carbon dioxide; some common ones are:
- Calvin-Benson cycle (cyanobacteria/chloroplasts), purple photosynthetic bacteria (proteobacteria), etc.
- reductive Krebs cycle (Chlorobium, etc.)
- reductive acetyl-CoA pathway (green nonsulfur bacteria like Chloroflexus)
- tapping into the methanogenesis reactions (methanogens)