Hydrogen-oxidizing bacteria are a group of facultative autotrophs that can use hydrogen as an electron donor. They can be divided into aerobes and anaerobes. The former use hydrogen as an electron donor and oxygen as an acceptor while the latter use sulphate or nitrogen dioxide as electron acceptors. Species of both types have been isolated from a variety of environments, including fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water. These bacteria are able to exploit the special properties of molecular hydrogen (for instance redox potential and diffusion coefficient) thanks to the presence of hydrogenases. The aerobic hydrogen-oxidizing bacteria are facultative autotrophs, but they can also have mixotrophic or completely heterotrophic growth. Most of them show greater growth on organic substrates. The use of hydrogen as an electron donor coupled with the ability to synthesize organic matter, through the reductive assimilation of CO2, characterize the hydrogen-oxidizing bacteria. Among the most represented genera of these organisms are Caminibacter, Aquifex, Ralstonia and Paracoccus. Sources of hydrogen Hydrogen is the most widespread element on our Earth, representing around three-quarters of all atoms. In the atmosphere, the concentration of molecular hydrogen (H2) gas is about 0.5–0.6 ppm, and so it represents the second-most-abundant trace gas after methane. H2 can be used as energy source in biological processes because it has a highly negative redox potential (E0′ = –0.414 V). It can be coupled with O2, in oxidative respiration (2H2 + O2 → 2H2O), or with oxidized compounds, such as carbon dioxide or sulfate. In an ecosystem, hydrogen can be produced through abiotic and biological processes. The abiotic processes are mainly due to geothermal production and serpentinization. In geothermal processes, hydrogen is usually present as a gas and may be obtained by different reactions: 1. Water may react with the silicon radical at high temperature: Si· + H2O → SiOH + H· H· + H· → H2 2. A proposed reaction between iron oxides and water may occur at temperatures higher than 800 °C: 2FeO + H2O → Fe2O3 + H2 2Fe3O4 + H2O → 3Fe2O3 + H2 Occurring at ambient temperature, serpentinization is an exothermic geochemical mechanism that takes place when ultramafic rocks from deep in the Earth rise and encounter water. This process can produce large quantities of H2, as well as methane and organic substances. The main biotic mechanisms that lead to the formation of hydrogen are nitrogen