Sulfur oxidation involves the oxidation of reduced sulfur compounds (such as sulfide ), inorganic sulfur (S), and thiosulfate () to form sulfuric acid ().  A classic example of a sulfur-oxidizing bacterium is Beggiatoa, a microbe originally described by Sergei Winogradsky, one of the founders of environmental microbiology. Another example is Paracoccus. Generally, the oxidation of sulfide occurs in stages, with inorganic sulfur being stored either inside or outside of the cell until needed.  This two step process occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate, allowing for a greater number of protons to be translocated across the membrane.  Sulfur-oxidizing organisms generate reducing power for carbon dioxide fixation via the Calvin cycle using reverse electron flow, an energy-requiring process that pushes the electrons against their thermodynamic gradient to produce NADH.  Biochemically, reduced sulfur compounds are converted to sulfite () and subsequently converted to sulfate () by the enzyme sulfite oxidase.  Some organisms, however, accomplish the same oxidation using a reversal of the APS reductase system used by sulfate-reducing bacteria (see above).  In all cases the energy liberated is transferred to the electron transport chain for ATP and NADH production.  In addition to aerobic sulfur oxidation, some organisms (e.g. Thiobacillus denitrificans) use nitrate () as a terminal electron acceptor and therefore grow anaerobically.