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Chemosynthesis is the biological conversion of 1-carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e.g. hydrogen gas, hydrogen sulfide) or methane as a source of energy, rather than sunlight, as in photosynthesis. Large populations of animals can be supported by chemosynthetic primary production at hydrothermal vents, methane clathrate cold seeps, and whale falls.

Many microorganisms in dark regions of the oceans use chemosynthesis to produce biomass from 1-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 between O₂ and substances such as hydrogen sulfide or ammonia. In this second case, the chemosynthetic microorganisms are dependent on photosynthesis which occurs elsewhere and which produces the O₂ that they require. Many chemosynthetic microorganisms are consumed by other organisms in the ocean, and symbiotic associations between chemosynthesizers and respiring heterotrophs are quite common.

It has been hypothesized that chemosynthesis may support life below the surface of Mars, Jupiter's moon Europa, and other planets.

Hydrogen sulfide chemosynthesis - CO₂+O₂+4{H₂S}→CH₂O¹+4{S}+3{H₂O}

¹note:CH₂O (carbohydrate) is what is used as food.