bacteria. Most grow best under microaerobic conditions because the hydrogenase enzyme is inhibited by the presence of oxygen and yet oxygen is still needed as a terminal electron acceptor in energy metabolism. The word Knallgas means "oxyhydrogen" (a mixture of hydrogen and oxygen, literally "bang-gas") in German. Strain MH-110 Ocean surface water is characterized by a high concentration of hydrogen. In 1989, an aerobic hydrogen-oxidizing bacterium was isolated from sea water. The MH-110 strain (aka DSM 11271, type strain of Hydrogenovibrio marinus) is able to grow under normal temperature conditions and in an atmosphere (under a continuous gas flow system) characterized by an oxygen saturation of 40% (analogous characteristics are present in the surface water from which the bacteria were isolated, which is a fairly aerated medium). This differs from the usual behaviour of hydrogen-oxidizing bacteria, which in general thrive under microaerophilic conditions (<10% O2 saturation). This strain is also capable of coupling the hydrogen oxidation with the reduction of sulfur compounds such as thiosulfate and tetrathionate. Metabolism Knallgas bacteria are able to fix carbon dioxide using H2 as their chemical energy source. Knallgas bacteria stand out from other hydrogen-oxidizing bacteria that, although using H2 as energy source, are not able to fix CO2, as Knallgas do. This aerobic hydrogen oxidation (H2 + O2 H2O), also known as the Knallgas reaction, releases a considerable amount of energy, having a ΔGo of –237 kJ/mol. The energy is captured as a proton motive force for use by the cell. The key enzymes involved in this reaction are the hydrogenases, which cleave molecular hydrogen and feed its electrons into the electron transport chain, where they are carried to the final acceptor, O2, extracting energy in the process. The hydrogen is ultimately oxidized to water, the end product. The hydrogenases are divided into three categories according to the type of metal present in the active site. These enzymes were first found in Pseudomonas saccharophila, Alcaligenes ruhlandii and Alcaligenese eutrophus, in which there are two types of hydrogenases: cytoplasmic and membrane-bound. While the first enzyme takes up hydrogen and reduces NAD+ to NADH for carbon fixation, the second is involved in the generation of the proton motive force. In most knallgas bacteria only the second is found. While these microorganisms are facultative autotrophs, some are also able to live heterotrophicically using organic substances as electron donors; in this case, the hydrogenase activity is less important