Mobilisation of metals in mineral tailings at the abandoned Sمo Domingos copper mine (Portugal) by indigenous acidophilic bacteria

The composition of microbial populations of tailings waste at the abandoned Sمo Domingos copper mine was investigated, and the abilities of pure and mixed cultures of indigenous microorganisms to leach metals from the tailings was examined. The indigenous acidophiles comprised Gram-negative sulfur-oxidising bacteria and Gram-positive bacteria that oxidised ferrous iron as well as sulfur. A mixed culture, obtained from the tailings by enrichment, was shown to leach virtually all the acid-extractable iron from the tailings when augmented with inorganic nutrients. In contrast, pure cultures of two bacteria from the tailings (a strain of Acidithiobacillus thiooxidans and an iron/sulfur-oxidising firmicute) were both ineffective at enhancing metal mobilisation from the mineral waste, in cultures amended with either inorganic or organic (yeast extract) nutrients. Mixed populations of isolates from the Sمo Domingos tailings were more effective at leaching the residual sulfide minerals in the tailings, but were again less effective than the mixed enrichment culture. Changes in microbial populations during bioleaching of mineral tailings were determined using terminal restriction fragment length polymorphism (T-RFLP) analysis, and by plating of samples on selective solid media. Both techniques confirmed that At. thiooxidans was present throughout culture incubation but, although the other bacteria identified were iron/sulfur-oxidising firmicutes, the exact identity of these strains varied with the technique used. The significance of these data in terms of the potential for re-processing the metal rich tailings, either to recover metals or to limit the time frame of acid mine drainage genesis, is discussed.