Every meal you eat started with an organism that feeds on light or rock
Autotrophs build their own food out of carbon dioxide, powered by sunlight or by chemical reactions with minerals. Plants, algae and vent-dwelling microbes sit at the base of every food chain, and the very first cells on Earth may have been autotrophs clustered around hot alkaline springs on the seafloor.
The German botanist Albert Bernhard Frank coined the word in 1892, drawing on the Greek for nourishment. An autotroph needs no living source of carbon or energy. It takes simple ingredients such as carbon dioxide and assembles carbohydrates, fats and proteins, spending part of the ATP it generates to turn NADP+ into NADPH along the way. Heterotrophs, which include all animals, almost all fungi and most bacteria, survive by eating those producers or each other.
There are two main routes. Photoautotrophs such as plants and cyanobacteria capture light, though plants can harness only about 1 percent of the solar energy that reaches them. Splitting water in the process releases the oxygen that other life breathes. Chemolithotrophs instead extract energy by oxidising inorganic compounds such as hydrogen sulfide, hydrogen gas, sulfur, ammonium or ferrous iron, and some can live entirely on atmospheric carbon dioxide and minerals. Many are archaea and bacteria at deep-ocean hydrothermal vents. In between sit mixotrophs, which take carbon from organic matter but power themselves with light or inorganic chemistry.
Stranger sources exist. Some fungi appear able to draw energy from ionising radiation, and such radiotrophic species were found growing inside a reactor at Chernobyl. In tropical streams, aquatic algae drive in-stream production at rates at least an order of magnitude greater than in comparable temperate systems.
Origins are debated, but one view holds that the first cells could not have lived off organic material delivered from space, which was too varied or chemically unsuitable. Instead they were probably heat-loving, oxygen-free chemolithoautotrophs at alkaline vents. Autotrophs likely appeared early in the Archean and spread during the Great Oxidation Event, when cyanobacteria ramped up oxygen-producing photosynthesis. The earliest photosynthetic bacteria used hydrogen sulfide, and scarcity of it pushed some to use water instead.
Source: Autotroph
