The Aquificalesare a group of thermophilic, chemolithotrophic and microaerophilic bacteria that are widespread in hydrothermal systemsReysenbach et al. (1994) Appl. Environ. Microbiol. 60:2113-2119; http://www.mbio.ncsu.edu/MB451•Aquifex pyrophilusis the “type strain”; a filamentous bacterium isolated from submarine hot sandy sediment at 106 m depth, Kolbeinsey Ridge north of Iceland. • Thermophilic, growing optimally at 85oC, up to 95oC; the most thermophilic bacterium in cultivation. • Microaerophilic - oxidizes H2, H2S, or S2O32-with O2or NO32-. • Based on 16S rRNA, they are the most deep branching lineage with the bacterial domain, but this placement remains controversial.• Most thermophiles are either anaerobes or microaerophiles - perhaps because the solubility of oxygen in water is very low at high temperatures. • Most are obligate chemoautotrophs and cannot grow heterotrophically; fix CO2via the reverse (reductive) TCA cycle. A few can also use acetate and formate as a C-source.