Concentrate mineralogy dictates the composition of bioleaching microbial consortia

Twelve mineral concentrates and one polymetallic ore were subjected to bioleaching by a heterogeneous mixed culture of moderately thermophilic prokaryotes in shake flask cultures incubated at 45°C. Dissolution of minerals was monitored, and the compositions of the consortia that developed in each case were determined using a combined cultivation-dependent (plating on selective solid media) and biomolecular (terminal restriction fragment length polymorphism) approach [1]. The concentrates used were divided into: (i) pyrite concentrates (two gold- and one cobalt-enriched); (ii) chalcopyrite (three concentrates, containing between 16 and 1500 g/t silver); (iii) copper-nickel (two concentrates); (iv) single concentrates of enargite; zinc (sphalerite) and lead (galena) and a copper-rich (predominantly chalcocite) black shale. The single ore sample (a polymetallic black schist) contained 0.3% nickel, 0.2% copper and 0.6% zinc. The cultures were inoculated with a physiologically and phylogenetically diverse range of moderately thermophilic acidophiles [2], comprising ten bacteria and one archaeon. These were: (i) iron- or sulfur-oxidizing autotrophs {Leptospirillum ferriphilum and At. caldus); (ii) iron-oxidizing mixotrophs/heterotrophs (Acidimicrobium ferrooxidans, a Ferroplasma sp. and "Ferrithrix thermotolerans" Y005); (iii) iron- and sulfur-oxidizing mixotrophs (four Firmicutes, including three Sulfobacillus spp. and one unclassified species); (iv) iron-reducing heterotrophic bacteria (Acidicaldus organivorans and Alicyclobacillus Y004). The bacteria and archaea were grown in appropriate liquid media [1] and inoculated into sterile mineral cultures in similar numbers. With the single exception of the enargite concentrate, all mineral samples were leached successfully by the moderately thermophilic consortium, often with >80% of the target metal(s) solubilised within 30 days. The compositions of the microbial communities that developed from the initial inocula were, however, highly variable (Table 1). In most cases, the established communities included both iron-and sulfur-oxidisers, and also both autotrophic and heterotrophic acidophiles. Notable exceptions were the chalcopyrite concentrates which were dominated (exclusively so in one case) by the mixotroph Sb. thermosulfidooxidans. Am. ferrooxidans was also frequently found to be an important (in terms of relative abundance) member of the effective bioleaching consortia, which is interesting as this iro-noxidiser has rarely been reported in bioleaching operations.