Interfacial insights of pyrite colonized by Acidithiobacillus thiooxidans cells under acidic conditions

Studies of interfacial processes involving leaching bacteria and sulfide minerals (MS) are necessary to understand and improve the bioleaching processes of mining industries. Interfacial studies of the role of extracellular polymeric substances (EPSs) during cell attachment to MS have been restricted mainly to iron-oxidizing bacteria, neglecting interfacial mechanisms associated with the biooxidation of reduced sulfur compounds (e.g., elemental sulfur, S0); nevertheless the reduced sulfur compounds may affect MS weathering or dissolution. The importance of sulfur-oxidizing microorganisms is evidenced by the fact that S0 was generally added to promote the growth of Acidithiobacillus thiooxidans in some bioleaching processes. Preliminary research coupled either epifluorescence and atomic force microscopy (AFM) or Raman spectroscopy and AFM to analyze biofilms of A. ferrooxidans on pyrite or the surface features of pyrite in biotic and abiotic experiments. Here, we applied the combination of AFM, Raman and principles of epifluorescence for the study of biofilms of the A. thiooxidans on previously oxidized pyrite. Our results showed that A. thiooxidans forms a monolayered biofilm, wherein the contact resulted in a strong adhesion force (467 pN) between the cells and the altered surface, perhaps due to an irreversible binding mechanism. The observations strongly suggested an intimate contact stage during the dynamic interfacial mechanisms of S0 biooxidation on the pyrite surface. During this process, an overproduction of EPS was recorded (ca. 100%), indicating that EPS plays a key role during microorganism/surface interactions.