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 sulfide) or methane as a source of energy, rather than sunlight, as in
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
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
whale falls, and
isolated cave water.
It has been hypothesized that chemosynthesis may support life below the surface of
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.