In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing
molecules (usually carbon dioxide or methane) and nutrients into organic matter using the
oxidation of inorganic compounds (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 also groups that include conspicuous or biogeochemically-important taxa include
the sulfur-oxidizing gamma and epsilon proteobacteria, the Aquificae, the methanogenic archaea and
the neutrophilic iron-oxidizing bacteria.
Many microorganisms in dark regions of the oceans 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.
It has been hypothesized that chemosynthesis may support life below the surface of Mars,
Jupiter's moon Europa, and other planets.
Chemosynthesis may have also been the first type of metabolism that evolved on Earth,
leading the way for cellular respiration and photosynthesis to develop later.
== Hydrogen sulfide chemosynthesis 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 + 12SInstead
of releasing oxygen gas while fixing carbon dioxide as in photosynthesis, hydrogen sulfide
chemosynthesis produces solid globules of sulfur in the process.
In bacteria capable of chemoautotrophy (a form a chemosynthesis), such as purple sulfur
bacteria, yellow globules of sulfur are present and visible in the cytoplasm.
== Discovery ==
In 1890, Sergei Nikolaevich Vinogradskii (or Winogradsky) proposed a novel type of life
process called "anorgoxydant".
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.
In 1897, Wilhelm Pfeffer coined the term "chemosynthesis" for the energy production by oxidation of
inorganic substances, in association with autotrophic carbon dioxide assimilation - what
would be named today as chemolithoautotrophy.
Later, the term would be expanded to include also chemoorganoautotrophs, which are organisms
that use organic energy substrates in order to assimilate carbon dioxide.
Thus, chemosynthesis can be seen as a synonym of chemoautotrophy.
The term "chemotrophy", less restrictive, would be introduced in the 1940s by André
Lwoff for the production of energy by the oxidation of electron donors, organic or not,
associated with auto- or heterotrophy.
=== Hydrothermal vents === The suggestion of Vinogradskii 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.
== See also == Primary nutritional groups
Autotroph Heterotroph
Photosynthesis Movile Cave