Many organisms are capable of using hydrogen () as a source of energy.  While several mechanisms of anaerobic hydrogen oxidation have been mentioned previously (e.g. sulfate reducing- and acetogenic bacteria), the chemical energy of hydrogen can be used in the aerobic Knallgas reaction:
:2 H + O → 2 HO + energy
In these organisms, hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping via electron transfer to various quinones and cytochromes.  In many organisms, a second cytoplasmic hydrogenase is used to generate reducing power in the form of NADH, which is subsequently used to fix carbon dioxide via the Calvin cycle.  Hydrogen-oxidizing organisms, such as Cupriavidus necator (formerly Ralstonia eutropha), often inhabit oxic-anoxic interfaces in nature to take advantage of the hydrogen produced by anaerobic fermentative organisms while still maintaining a supply of oxygen.