Fig. 4

Enzymatic pathways for oxidation of electrochemically reduced thiosulfate. Although challenging, it may be possible to electrochemically reduce sulfate to thiosulfate (we have placed a ? at the sulfate to thiosulfate reaction to indicate this difficulty). Thiosulfate can be directly oxidized to sulfate through the Sox system (a). Also, a membrane-bound, the thiosulfate:quinone oxidoreductase (TQO) can oxidize thiosulfate to tetrathionate in a 2-electron reaction (b), followed by oxidation to sulfate through the tetrathionate oxidation pathways shown in Fig. 3. This cycle is completed when sulfate is electrochemically reduced back to thiosulfate at the cathode