An autotroph (in Greek eauton = self and trophe = nutrition) is an organism that produces its own cell mass and organic compounds from carbon dioxide as sole carbon source, using either light or chemical compounds as a source of energy. Plants and other organisms using photosynthesis are photolithoautotrophs; bacteria that utilize the oxidation of inorganic compounds such as hydrogen sulfide or ferrous iron for energy conservation are chemolithoautotrophs.

Autotrophs are a vital part of the food chain. They take energy from the sun or from inorganic sources and convert it into a form (organic molecules) that they use to carry out biological functions including cell growth; and that other organisms (called heterotrophs) can utilize as food. Thus, heterotrophs — animals, fungi, as well as most bacteria and protozoa — depend on autotrophs for energy and for the raw materials to make complex organic molecules. The heterotrophs obtain energy by breaking down organic molecules in their food. Even carnivorous animals rely on autotrophs because the energy and organic building blocks obtained from their prey comes ultimately from the autotrophs eaten by the prey.