[2] 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 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.
A proposed reaction between iron oxides and water may occur at temperatures higher than 800 °C: 2FeO + H2O → Fe2O3 + H2 2Fe3O4 + H2O → 3Fe2O3 + H2 [6] 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.
[11] The second mechanism, fermentation, is performed by some anaerobic heterotrophic bacteria, in particular Clostridia,[12] that degrade organic molecules, producing hydrogen as one of the products.
This type of metabolism mainly occurs in anoxic sites, such as lake sediments, deep-sea hydrothermal vents and the animal gut.
In this environment hydrogen oxidation represents a significant origin of energy, sufficient to conduct ATP synthesis and autotrophic CO2 fixation, so hydrogen-oxidizing bacteria form an important part of the ecosystem in deep sea habitats.
Among the main chemosynthetic reactions that take place in hydrothermal vents, the oxidation of sulphide and hydrogen holds a central role.
The MH-110 strain (aka DSM 11271, type strain of Hydrogenovibrio marinus[23][24]) 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).
[25][26] This strain is also capable of coupling the hydrogen oxidation with the reduction of sulfur compounds such as thiosulfate and tetrathionate.
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.
[31] 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 or completely absent.
[34][35] Given enough nutrients, H2, O2 and CO2, many knallgas bacteria can be grown quickly in vats using only a small amount of land area.