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Chemosynthesis
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In biochemistry, chemosynthesis is the biological conversion of one or more
carbon molecules (usually carbon dioxide or methane) and nutrients into
organic matter using the oxidation of inorganic (e.g. hydrogen gas, hydrogen
sulfide) or methane as a source of energy, rather than sunlight, as in
photosynthesis. Chemoautotrophs, organisms that obtain carbon through
chemosynthesis, are phylogenetically diverse, but groups that include
conspicuous or biogeochemically-important taxa include the sulfur-oxidizing
gamma and epsilon proteobacteria, the aquificaeles, the methanogenic archaea
and the neutrophilic iron-oxidizing bacteria.
Many microorganisms in dark regions of the oceans also use chemosynthesis to
produce biomass from single carbon molecules. Two categories can be
distinguished. In the rare sites at which hydrogen molecules (H2) are available,
the energy available from the reaction between CO2 and H2 (leading to
production of methane, CH4) can be large enough to drive the production of
biomass.
Alternatively,
in
most
oceanic
environments,
energy
for
chemosynthesis derives from reactions in which substances such as hydrogen
sulfide or ammonia are oxidized. This may occur with or without the presence
of oxygen.
Many chemosynthetic microorganisms are consumed by other organisms in the
ocean, and symbiotic associations between chemosynthesizers and respiring
heterotrophs are quite common. Large populations of animals can be supported
by chemosynthetic secondary production at hydrothermal vents, methane
clathrates, cold seeps, whale falls, and isolated cave water.
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It has been hypothesized that chemosynthesis may support life below the
surface of Mars, Jupiter's moon Europa, and other planets.
Process
Giant tube worms use bacteria in their trophosome to fix carbon dioxide (using
hydrogen sulfide as an energy source) and produce sugars and amino acids.
Some reactions produce sulfur:
hydrogen sulfide chemosynthesis:
12H2S + 6CO2 → C6H12O6 (=carbohydrate) + 6H2O + 12S
instead of releasing oxygen gas as in photosynthesis, the process produces solid
globules of sulfur. In bacteria capable of chemosynthesis, such as purple sulfur
bacteria[citation needed], yellow globules of sulfur are present and visible in the
cytoplasm.
Discovery
In 1890, Sergei Nikolaevich Vinogradskii (or Winogradsky) proposed a novel
life process called chemosynthesis. His discovery suggested that some microbes
could live solely on inorganic matter and emerged during his physiological
research in the 1880s in Strassburg and Zurich on sulfur, iron, and nitrogen
bacteria.
Hydrothermal vents
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This was confirmed nearly 90 years later, when hydrothermal ocean vents were
predicted to exist in the 1970s. The hot springs and strange creatures were
discovered by Alvin, the world's first deep-sea submersible, in 1977 at the
Galapagos Rift. At about the same time, Harvard graduate student Colleen
Cavanaugh proposed chemosynthetic bacteria that oxidize sulfides or elemental
sulfur as a mechanism by which tube worms could survive near hydrothermal
vents. Cavanaugh later managed to confirm that this was indeed the method by
which the worms could thrive, and is generally credited with the discovery of
chemosynthesis.
A 2004 television series hosted by Bill Nye named chemosynthesis as one of
the 100 greatest scientific discoveries of all time.
Oceanic crust
In 2013, researchers reported their discovery of bacteria living in the rock of the
oceanic crust below the thick layers of sediment, and apart from the
hydrothermal vents that form along the edges of the tectonic plates. Preliminary
findings are that these bacteria subsist on the hydrogen produced by chemical
reduction of olivine by seawater circulating in the small veins that permeate the
basalt that comprises oceanic crust. The bacteria synthesize methane by
combining hydrogen and carbon dioxide.
Use of term in molecular nanotechnology
The term chemosynthesis is also used in molecular nanotechnology broadly to
refer to any chemical synthesis where reactions occur due to random thermal
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motion, a class which encompasses almost all of modern synthetic chemistry.
The human-authored processes of chemical engineering are accordingly
represented as biomimicry of the natural phenomena above, and the entire class
of non-photosynthetic chains by which complex molecules are constructed is
described as chemo-.
This form of engineering is then contrasted with mechanosynthesis, a
hypothetical process where individual molecules are mechanically manipulated
to control reactions to human specification. Since photosynthesis and other
natural processes create extremely complex molecules to the specifications
contained in RNA and stored long-term in DNA form, advocates of molecular
engineering claim that an artificial process can likewise exploit a chain of longterm storage, short-term storage, enzyme-like copying mechanisms similar to
those in the cell, and ultimately produce complex molecules which need not be
proteins. For instance, sheet diamond or carbon nanotubes could be produced by
a chain of non-biological reactions that have been designed using the basic
model of biology.
Use of the term chemosynthesis reinforces the view that this is feasible by
pointing out that several alternate means of creating complex proteins, mineral
shells of mollusks and crustaceans, etc., evolved naturally, not all of them
dependent on photosynthesis and a food chain from the sun via chlorophyll.
Since more than one such pathway exists to creating complex molecules, even
extremely specific ones such as proteins edible to fish, the likelihood of humans
being able to design an entirely new one is considered (by these advocates) to
be near certainty in the long run, and possible within a generation.
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