Maximum copper dissolution was reached when at least one fraction of the inoculated microorganisms were able to approach chalcopyrite surface. In this case, notably, the fraction of the population of microorganisms that were prevented from reaching chalcopyrite was still observed to increase. In experiments where all the inoculated microorganisms were prevented from reaching chalcopyrite, the cell population did not grow. In this case copper dissolution was only 50% of the amount reached when full bacterial contact was allowed, and was similar to the amount of copper dissolved at aerated-abiotic conditions. HPLC results on the analysis of dissolved sulfur species showed that when microorganisms were able to reach chalcopyrite there was accumulation in solution of thiosulfate (S2O3)(-2) and sulfite (SO3)(-2), in addition to the formation of bisulfite (HSO3)(-), bisulfate (HSO4)(-) and sulfate (SO4)(-2) which was also observed in aerobic-abiotic conditions. It could be concluded that in bioleaching of chalcopyrite in the presence of S. metallicus there is a cooperative action between attached cells which can oxidize sulfur-containing surface layers on chalcopyrite, forming thiosuffate, sulfite and bisulfite, and planktonic cells which further oxidize these intermediate compound to bisulfate and sulfate. Removal of surface passivating layers under the catalytic action of attached microorganisms is a key catalytic factor as it greatly enhances the oxidative action of ferric iron on chalcopyrite. (C) 2008 Elsevier B.V. All rights reserved.