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A comprehensive monitoring program has been performed at an industrial bioleaching heap at the Escondida mine in Chile since 2006, in order to study possible changes to the indigenous microbial population. In the initial stage of the bioleaching heap operation, the microbial population was dominated by mesophilic microorganisms. When the height of the heap increased, the microbial population changed from predominantly mesophilic to thermotolerant and moderate thermophilic microorganisms. The results of molecular analyses indicated that under these conditions Leptospirillum ferriphilum and Sulfobacillus thermosulfidooxidans are the most abundant microorganisms. Those results obtained show that the highest ferrous iron oxidation activity at the third lift occurs at the same operational temperature window as that of the thermotolerant and moderate thermophiles. An increase in the expression of genes (including rfa F1) associated with cell envelope biogenesis was indicative of the active growth of L. ferriphilum . We conclude that the increasing heap height and the consequent increase in temperature within the heap triggered a change in the microbial community from mesophilic to moderate thermophilic microorganisms. Further, this shift had a relevant impact on the metallurgical performance of the industrial bioheap process at Escondida mine. [ABSTRACT FROM AUTHOR]