 Chemotrophs
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.
 Complex
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.
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