Aquifex

Aquifex is a genus of Gram-negative, hyperthermophilic bacteria in the phylum Aquificae. Members are chemolithoautotrophs that obtain energy by oxidizing inorganic compounds and fix carbon dioxide into biomass. The best-known species is Aquifex aeolicus; another described species is Aquifex pyrophilus. Aquifex organisms are found in extremely hot environments, such as terrestrial hot springs and deep-sea hydrothermal vents, where reduced compounds such as hydrogen and sulfide are available.

Metabolically, Aquifex species typically use molecular hydrogen as an electron donor and, when oxygen is available, use it as the terminal electron acceptor. They fix CO2 via the reverse tricarboxylic acid cycle, a carbon fixation pathway thought to be ancient among autotrophs. This metabolism places Aquifex among the early-branching autotrophic bacteria and makes it a model for studying ancient metabolism and thermophily.

Taxonomically, Aquifex is the type genus of the phylum Aquificae and includes a small number of described species. The genome of Aquifex aeolicus has been sequenced, and studies emphasize its compact genome and adaptations to high-temperature environments.

See also: Aquificae, reverse TCA cycle, chemolithoautotrophy, early evolution of bacteria.