Catalyzes the oxidation of 2 molecules of thiosulfate to tetrathionate. 2 Fe(III)-[cytochrome c] + 2 thiosulfate = 2 Fe(II)-[cytochrome c] + 2 H(+) + tetrathionate Optimum temperature is 20 degrees Celsius. Monomer. Binds 2 heme c groups covalently per subunit. Cells are unable to produce tetrathionate from thiosulfate.