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- Role of pure and mixed cultures of gram-positive eubacteria in mineral leachingPublication . Johnson, D. Barrie; Nicolau, Paula Bacelar; Okibe, Naoko; Yahya, Adibah; Hallberg, Kevin B.Research on the biooxidation of sulfidic minerals has tended to be heavily biased towards Gram-negative bacteria, such as Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans. Currently, just three species of mineral-oxidising Gram-positive bacteria are recognised: Sulfobacillus thermosulfidooxidans, Sulfobacillus acidophilus and Acidimicrobium (Am.) ferrooxidans, all of which are thermotolerant prokaryotes. We have isolated and characterised a number of phylogenetically distinct Gram-positive ironmetabolising bacteria, including mesophilic and moderately thermophilic strains. Mesophilic isolates include (i) novel Sulfobacillus spp., some of which are the more acidophilic than all known iron-oxidising bacteria, (ii) "Ferrimicrobium acidiphilum", an actinobacterium most closely related to Am. ferrooxidans, and (iii) a group of low GC Gram-positives which appear to represent a novel genus. Moderately thermophilic isolates include a novel Sulfobacillus sp., an Alicyclobacillus spp. that, in contrast to currently recognised species, grows anaerobically by reduction of ferric iron and thrives in mineral leaching environments, and a new genus/species of iron- and sulfur-oxidising bacterium with the proposed name "Caldibacillus ferrivorus". These novel prokaryotes exhibited varying degrees of mineral leaching efficiencies, with the mesophilic Sulfobacillus spp. being particularly adept at solubilising pyrite at very low (<1) pH values. All novel Gram-positive isolates catalysed the oxidative dissolution of pyrite at lower redox potentials than Gram-negative mesophiles. Mixed cultures of Gram-positive bacteria, either with other Gram-positives or with Gram-negative bacteria, were often more effective mineral-leaching systems than corresponding pure cultures.
- Leaching of pyrite by acidophilic, heterotrophic, iron-oxidizing bacteria in pure and mixed culturePublication . Nicolau, Paula Bacelar; Johnson, D. BarrieSeven strains of heterotrophic iron-oxidizing acidophilic bacteria were examined to determine their abilities to promote oxidative dissolution of pyrite (FeS2) when they were grown in pure cultures and in mixed cultures with sulfur-oxidizing Thiobacillus spp. Only one of the isolates (strain T-24) oxidized pyrite when it was grown in pyrite-basal salts medium. However, when pyrite-containing cultures were supplemented with 0.02% (wt/vol) yeast extract, most of the isolates oxidized pyrite, and one (strain T-24) promoted rates of mineral dissolution similar to the rates observed with the iron-oxidizing autotroph Thiobacillus ferrooxidans. Pyrite oxidation by another isolate (strain T-21) occurred in cultures containing between 0.005 and 0.05% (wt/vol) yeast extract but was completely inhibited in cultures containing 0.5% yeast extract. Ferrous iron was also needed for mineral dissolution by the iron-oxidizing heterotrophs, indicating that these organisms oxidize pyrite via the “indirect” mechanism. Mixed cultures of three isolates (strains T-21, T-23, and T-24) and the sulfur-oxidizing autotroph Thiobacillus thiooxidans promoted pyrite dissolution; since neither strains T-21 and T-23 nor T. thiooxidans could oxidize this mineral in yeast extract-free media, this was a novel example of bacterial synergism. Mixed cultures of strains T-21 and T-23 and the sulfur-oxidizing mixotroph Thiobacillus acidophilus also oxidized pyrite but to a lesser extent than did mixed cultures containing T. thiooxidans. Pyrite leaching by strain T-23 grown in an organic compound-rich medium and incubated either shaken or unshaken was also assessed. The potential environmental significance of iron-oxidizing heterotrophs in accelerating pyrite oxidation is discussed.