Fig. 3

Enzymatic pathways for oxidation of electrochemically reduced tetrathionate. Tetrathionate (S₄O₆^2-) is oxidized by a membrane-bound Tetrathionate hydrolase (TTH) to sulfate and thioperoxymonosulfate (S₃O₃^2-) which spontaneously dissociates into sulfur (S⁰) and thiosulfate (S₂O₃^2-). (a) Thiosulfate is oxidized via the Sox pathway, similar to that shown in Fig. 2a. However, an additional oxidation step, catalyzed by SoxB at the beginning of the pathway, releases an additional sulfate molecule, that can also be recycled back to tetrathionate via cathode reduction. (b) Elemental sulfur is converted to sulfide by the Dissimilatory sulfite reductase (Dsr), then following the pathway shown is Fig. 2b, sulfide is oxidized to sulfate. This cycle is completed when sulfate is electrochemically reduced back to tetrathionate at the cathode