Autotrophs are organisms that produce biomass de novo, and heterotrophs are organisms that consume biomass, alive or dead. Autotrophs are primary producers, which fix carbon into carbohydrate with energy from largely inorganic sources. The two kinds of autotrophs are chemo-autotrophs and photoautotrophs. The former are bacteria and Archaea that use compounds such as methane or reduced sulfur, nitrogen, or metals as an energy source. Many chemoautotrophs live in extreme environments such as hot springs and hydrothermal vents of the deep sea, but others such as nitrifying bacteria are common in soils, lakes, and the sea. Evolutionary research suggests that some bacterial chemoautotrophs are the closest living descendants of the original cellular organisms. It is not yet known if the original cellular organisms were chemoauto-trophic - and derived energy from prebiotic reduced substances - or were photoautotrophic. However, since
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