 Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H2.Hydrogen gas is a substrate for methanogens and sulfate reducers, which keep the concentration of hydrogen low and favor the production of such an energy-rich compound, but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus.For example, "Clostridium pasteurianum" ferments glucose to butyrate, acetate, carbon dioxide, and hydrogen gas