 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.