The discovery of hydrogenases in hyperthermophiles has important ramifications not only in microbial physiology and evolution but also in biotechnologies. These organisms are the source of extremely stable enzymes (regarding temperature, pressure, and 02). Aquifex aeolicus is a microaerophilic, hyperthermophilic bacterium containing three [NiFe] hydrogenases. It is the most hyperthermophilic bacterium known to date and grows at 85 degrees C under a H-2/CO2/O-2 atmosphere. The Aquificales represent the earliest branching order of the bacterial domain indicating that they are the most ancient bacteria. Two Aquifex hydrogenases (one membrane-bound and one soluble) have been purified and characterized. In contrast to the majority of the [NiFe] hydrogenases, the hydrogenases from A. aeolicus are rather tolerant to oxygen. The molecular basis of the oxygen resistance of Aquifex hydrogenases has been investigated. The great stability of Aquifex hydrogenases with respect to oxygen and high temperatures make these enzymes good candidates for biotechnological uses. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.