The phylum Aquificae 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 and 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. They are gram negative.