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Hydrogen oxidizing bacteria are a group of facultative autotrophs that can use hydrogen as electron donor.

They can be divided into aerobics and anaerobics. The former use hydrogen as an electron donor and oxygen as an acceptor while the latter use sulphate or nitrogen dioxide as electron acceptors. Some species of both bacteria types have been isolated in different environments, for example in fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water.

These organisms 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 mutter, through the reductive assimilation of CO2, characterize the hydrogen oxidizing bacteria. Among the most represented genres of these organisms we find: Caminibacter, Aquifex, Ralstonia and Paracoccus.

Strain MH-110
Ocean’s surface water is characterised by a high concentration of hydrogen. In 1989, for the first time, an aerobic hydrogen oxidizing bacteria was isolated from sea water and the discovery of this strain was very important also because for the first time a hydrogen oxidizing bacteria was identified in normal temperature condition. Experimentally it has been shown that the MH-110 strain is able to grow in an atmosphere (under a continuous gas flow system) characterized by an oxygen concentration of 40% (analogue characteristics are present in the surface water from which the bacteria were isolated which is, in fact, a fairly aerated region). This differs from the usual behaviour of hydrogen oxidizing bacteria which in general thrive strictly under microaerophilic conditions (<10% O2).

This strain is also capable to couple the Hydrogen oxidation with the reduction of sulfur compounds such as thiosulfate and tetrathionate.

Hydrogen oxidizing bacteria in Hydrothermal vents
H2 is an important electron donor in a particular environment: Hydrothermal vents. In this environment hydrogen oxidation represents a significant origin of energy, sufficient to conduct ATP synthesis and autotrophic CO2 fixation so hydrogen oxidizing bacteria are relevant in deep sea habitats. Among the main chemosynthetic reactions that take place in hydrothermal vents, the oxidation of the sulphide and the hydrogen one covers a central role. In particular, for autotrophic carbon fixation, hydrogen oxidation metabolism is more favored than the sulfide/thiosulfate oxidation, although less energy is released (only -237 kJ/mol compared to – 797 kJ/mol). To fix a mole of carbon during the hydrogen oxidation, one third of the energy necessary for the sulphide oxidation is used. This is due to the redox potential of hydrogen, which is more negative than NAD (P)/H. Based on the amount of sulphide, hydrogen and other farm biotics, this phenomenon can be intensified leading, in some cases, to an energy production by oxidation of the hydrogen of 10 -18 times higher than produced one by the sulphide oxidation.