Chemolithotrophy

Chemolithotrophy is a form of metabolism in which organisms obtain energy by oxidation of inorganic chemical compounds. Chemolithotrophs can be bacteria or archaea. The process contrasts with chemoorganotrophy, where organic compounds serve as the energy source, and with phototrophy, which uses light as an energy source.

In chemolithotrophy, energy is released during the transfer of electrons from inorganic electron donors, such as molecular hydrogen (H2), reduced sulfur compounds (like hydrogen sulfide and elemental sulfur), ferrous iron (Fe2+), or ammonia (NH3) and nitrite (NO2-). The electrons are transferred to an electron acceptor, most commonly oxygen in aerobic conditions, but many chemolithotrophs use alternative acceptors such as nitrate, nitrite, sulfate, or ferric iron under anaerobic or microaerophilic conditions. The overall redox reaction provides energy for cellular processes.

Most chemolithotrophs are autotrophs, fixing carbon dioxide to build biomass. They employ carbon fixation pathways such as the Calvin cycle, the reverse tricarboxylic acid cycle, or alternative routes, using the energy from inorganic oxidation to drive incorporation of CO2. Some chemolithotrophs can also assimilate organic carbon, in which case they are described as mixotrophs or heterotrophic under certain conditions.

Chemolithotrophs are widespread in a variety of environments, including soils, freshwater and marine systems, hydrothermal vents, and sulfur-rich springs. They play crucial roles in biogeochemical cycles, particularly the nitrogen, sulfur, and iron cycles, and they can support primary production in ecosystems where light is unavailable.

Historically, chemolithotrophy was uncovered in the late 19th and early 20th centuries by researchers such as Sergei Winogradsky. Today, diverse chemolithotrophic lineages are known among bacteria and archaea, illustrating the metabolic diversity that underpins inorganic energy acquisition.