Final Report Summary - MICROIPB (Microbial activities of the sulfur cycle in the subsurface of the Iberian Pyritic Belt) The Iberian Pyrite Belt (IPB), located in southwestern Spain is one of the largest sulfide deposits on Earth. It is of tremendous interest due to its high concentrations of metals (As, Fe, Cu, Zn, Mn, Cr, Ni ..), and constant low pH (mean 2.3) in ground and surface waters. These conditions have largely been attributed to mining practices over the last 5,000 years. However, recent evidence has shown that microbes may play a large role in the production and maintenance of these conditions. This study sought to elucidate the role microbes play in the in various environments in the IPB, and the effect of their activities on water geochemistry and geology. To accomplish these objectives, we obtained samples from three distinct environments of the IPB: The subsurface down to 600 m, 3 acidic lakes, and the acidic Tinto river. Samples recovered from the IPB were subjected to detailed biological, geological and chemical analysis to determine the main biogeochemical transformations within these environments. From our results, we were able to conclude that some of the main biogeochemical processes occurring in the IPB: (1) oxidation of sulfur containing minerals, (2) iron reduction, (3) iron oxidation (4) and sulfate reduction. We were also able to identify various groups of microbes capable of carrying out these transformations. Overall, the activities of these organisms contributed to the formation and dissolution of minerals, and the maintenance of water quality in various environmental settings of the IPBSL. The results of this project are expected to contribute to our understanding of the effect of microbial processes in acidic environments.