The Aquificae phylum is a diverse collection of bacteria that live in harsh environmental settings. They have been found in hot springs, sulfur pools, and thermal ocean vents. Members of the genus Aquifex, for example, are productive in water between 85 to 95 °C. They are the dominant members of most terrestrial neutral to alkaline hot springs above 60 degrees celsius. They are autotrophs, and are the primary carbon fixers in these environments. They are true bacteria (domain bacteria) as opposed to the other inhabitants of extreme environments, the Archaea.
There is currently no consensus regarding the taxonomy of genera within Aquificae. One standard text claims that only the genera Aquifex, Calderobacterium, Hydrogenobacter, and Thermocrinis belong in the Aquificales order. Another claims that, in addition to genera within the Aquificaceae and Hydrogenothermaceae families, the following genera are incertae sedis (unclassified), but within Aquificae: Balnearium, Desulfurobacterium, EX-H1 group, and Thermovibrio.
Cavalier-Smith has postulated that Aquificae is part of the Proteobacteria.
Along with Thermotogae, members of Aquificae are thermophilic eubacteria.