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An autotroph (from the Greek autos = self and trophe = nutrition) is an organism that produces complex organic compounds from simple 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).