A composite model of sulfur oxidation electron transport pathway of At. ferrooxidans based on references [10, 76, 96]. Thiol groups of outer membrane proteins are believed to transport the sulfur to the periplasm where it is oxidized by a periplasmic sulfur dioxygenase (SDO) to sulfite and a sulfite acceptor oxidoreductase (SOR) to sulfate . Although other cytochrome oxidases are present, a ba3 cytochrome oxidase and a bc1 II complex together with a bd-type ubiquinol oxidase are believed to play the major roles during sulfur oxidation [10, 96]. Rusticyanin and an iron oxidizing protein (not shown) might also be involved during sulfur oxidation but their exact role is still to be determined .