Aquificales are metabolically versatile chemolithoautotrophic thermophilic bacteria. This group is widespread in both deep-sea and terrestrial hydrothermal systems. In Yellowstone National Park, they were first described in early descriptions of the biology of the park, and later captured the attention of many microbiologists including Brock, Stahl, Pace, and others. There are four genera currently described from Yellowstone: Thermocrinis, Sulfurihydrogenibium, Hydrogenobacter and the only acidophilic genus, Hydrogenobaculum. Aquificales appear to fix CO2 using the reductive TCA cycle, although several species can also obtain carbon from organic sources such as acetate and formate. Hydrogen and sulfur appear to be the preferred electron donors. Furthermore, many of these filamentous bacteria are associated with visible iron and sulfur mineral precipitation, which points to their overall importance in biogeochemical cycling in hot spring ecosystems.