chemoautotroph

Chemoautotrophs are organisms that obtain their energy by oxidizing inorganic chemical compounds (chemolithotrophs) and use carbon dioxide or bicarbonate as their sole carbon source, enabling them to synthesize organic matter from inorganic carbon without relying on photosynthesis. They are a key component of chemolithotrophic metabolism.

Energy sources and electron donors for chemoautotrophs include hydrogen (H2), hydrogen sulfide (H2S), ferrous iron (Fe2+), ammonia (NH3/NH4+), and other reduced inorganic compounds. Electron acceptors can be oxygen in aerobic species or alternative electron acceptors such as nitrate or sulfate in anaerobic environments. This metabolic flexibility allows chemoautotrophs to inhabit diverse habitats, including oxic, microaerophilic, and anoxic conditions.

Carbon fixation pathways used by chemoautotrophs vary. The Calvin cycle is common, particularly among many bacteria and some archaea. Other organisms employ pathways such as the reverse tricarboxylic acid (TCA) cycle or the 3-hydroxypropionate cycle, among others, to assimilate CO2 into organic molecules.

Taxonomic distribution and ecology: chemoautotrophy is found in prokaryotes, including bacteria and archaea. They are especially important in environments lacking light, such as deep-sea hydrothermal vents, sulfur springs, and oxic or suboxic soils, where they form the base of microbial food webs. They also play roles in biogeochemical cycles, including the sulfur, nitrogen, and iron cycles, by driving primary production and nutrient transformations.

Examples include sulfur-oxidizing bacteria such as Thiomargarita namibiensis and nitrifying bacteria like Nitrosomonas (ammonia-oxidizers) and Nitrobacter (nitrite-oxidizers), as well as sulfur-oxidizing archaea such as Sulfolobus.