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Archaea and Evolution

Grand Prismatic Spring at Hot Springs Yellowstone National ParkChemotrophs are organisms that obtain energy by the oxidation of electron donors. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). In contrast, phototrophs utilize solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemoautotrophs utilize inorganic energy sources, of which hydrogen sulfide, elemental sulfur, ferrous iron, molecular hydrogen, and ammonia are prominent examples. Most are bacteria or archaea that live, for example, in the hostile environments seen around deep sea vent, hot springs, volcanic fumaroles and geysers. Chemosynthetic archaea form the base of the food chain, supporting diverse organisms, including tube worms, clams, and shrimp. Evolutionary biologists posit that the earliest organisms on Earth were chemoautotrophs that produced oxygen as a by-product and later evolved into both aerobic, animal-like organisms and photosynthetic, plant-like organisms. Chemoheterotrophs utilize inorganic energy from sources such as sulfur and chemoorganoheterotrophs utilize organic energy sources such as carbohydrates, lipids, and proteins.

Deep Sea VentsComplex biological communities are found near deep-ocean vents as shown above left. The comprising organisms depend on chemosynthetic bacteria for food. The water that flows out of the hydrothermal vent is rich in dissolved minerals that support a large population of chemo-autotrophic bacteria. These bacteria use sulfur compounds, particularly hydrogen sulfide, a chemical highly toxic to most known organisms, to produce organic material through the process of chemosynthesis. The chemosynthetic bacteria grow into a thick mat that are directly eaten byamphipods and copepods. These, in turn, are consumed by bigger animals such as snails, shrimp, crabs, tube worms, fishes, and octopuses to form a complex predator and prey food chain. The main families of organisms found around seafloor vents are annelids, pogonophorans, gastropods, and crustaceans, with large bivalves, vestimentiferan worms, and "eyeless" shrimp making up the preponderance of life in the vent ecosystem.

Iron–sulfur world theory posits a Hydrothermal origin of life, that is, life might have originated at hydrothermal vents. The theory proposes that an early form early chemical metabolism predated genetics. More specifically, amino-acid synthesis could have occurred deep in the Earth's crust and that these amino-acids were subsequently shot up along with hydrothermal fluids into cooler waters, where lower temperatures and the presence of clay minerals would have fostered the formation of peptides and protocells. The theory is consistent with the abundance of methane and ammonia present in hydrothermal vents, an environment that was not provided by the Earth's primitive atmosphere. A major limitation to this hypothesis is the lack of stability of organic molecules at high temperatures, but some have suggested that life would have originated outside of the zones of highest temperature. There are numerous species of extremophiles and other organisms currently living immediately around deep-sea vents, suggesting that this was, indeed, the so-called primordial soup of life.