Abstract:A bioelectrochemical study of charge transfer in the biofilm-chalcopyrite interface was performed to investigate the effect of surficial reduced sulfur species (RSS), in the form of non-stochiometric compounds or polysulfides (S n 2− ) and elemental sulfur (S 0 ) on a biofilm structure, during the earliest stages (1, 12 and 24 h) of chalcopyrite biooxidation by Acidithiobacillus thiooxidans alone and adding Leptospirillum sp. The surface of massive chalcopyrite electrodes was exposed to the bacteria, which were analyzed electrochemically, spectroscopically, and microscopically. At the studied earlier times, charge transfer and significant differences in the biofilm structure were detected, depending on the presence of Leptospirillum sp. acting on A. thiooxidans biofilms. Such differences were a consequence of a continuous chalcopyrite pitting and promoting changes in biofilm hydrophobicity. A. thiooxidans modifies the reactive properties of RSS and favors an acidic dissolution, which shifts into ferric dissolution when Leptospirillum sp. is present. A. thiooxidans allows H + and Fe 3+ diffusion, and Leptospirillum sp. enables to surpass the charge transfer (reactivity) barrier between the mineral interface and the ions. The observed changes of hydrophobicity on the interface are associated to ions and electrons activity and transfer. Finally, a model of S 0 biooxidation by A. thiooxidans alone or with Leptospirillum sp. is proposed.