 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).