An autotroph (from the Greek autos = self and trophe = nutrition) is an organism that produces complex organic compounds from simple inorganic molecules and an external source of energy, such as light or chemical reactions of inorganic compounds. Autotrophs are considered producers in a food chain. Plants and other organisms that carry out photosynthesis are phototrophs (or photoautotrophs). Bacteria that utilize the oxidation of inorganic compounds such as hydrogen sulfide, ammonium or ferrous iron as an energy source are chemoautotrophs (some are known as lithotrophs). Autotrophs are a vital part of the food chains of all plant ecosystems. They take energy from the environment (sunlight or inorganic sources) and use it to process carbon-based and other organic molecules that are used to carry out various biological functions such as cell growth. Other organisms, called heterotrophs, utilize autotrophs as food to carry out these same functions. Thus, heterotrophs — animals, fungi, as well as most bacteria and protozoa — depend on autotrophs for both energy and raw materials to make complex organic molecules. This mechanism is called primary production in the sea. Heterotrophs obtain energy by breaking down organic molecules obtained in food. Carnivorous animals ultimately rely on autotrophs because the energy and organic building blocks obtained from their prey comes from autotrophs they preyed upon. There are some species of organisms that require organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy. Such organisms are not defined as autotrophic, but rather as heterotrophic. An organism that obtains carbon from organic compounds but obtains energy from light is called a photoheterotroph, while an organism that obtains carbon from organic compounds but obtains energy from the oxidation of inorganic compounds is termed a chemoheterotroph.
