Role of Thiobacillus and sulfate-reducing bacteria in iron biocycling in oxic and acidic mine tailings

The tailings of the abandoned Kam Kotia mine (Cu–Zn ore) located near Timmins, Ontario, Canada, were studied to understand the role of bacteria in Fe cycling. Tailings samples taken along a 70 cm deep profile were oxidized and acidic (pH 2–4). The release of large concentrations of dissolved Fe and SO₄²⁻ in the surface pore waters coincided with the presence of large populations of Thiobacillus, an acidophilic iron-oxidizing bacterium. The chemical and microbial oxidation by Thiobacillus of Fe-sulfides was extensive in the Kam Kotia tailings and corresponded to a near depletion of the pyrite content of the tailings around the same depth. Concurrently, de novo biomineralization occurred within the tailings as indicated by the enrichment of Fe-oxide minerals close to the tailings bacteria. Bacteria, such as Thiobacillus, provided binding sites for dissolved metal species and served as nucleation surfaces for the development of Fe-rich minerals under acidic conditions. Sulfate-reducing bacteria (SRB) were also recovered in the tailings, in the lower portion of the profile where slightly oxidizing and acidic (pH 3–4) conditions prevailed. SRB possibly survived in microenvironments having more reduced and alkaline conditions because they did not tolerate oxidizing and acidic conditions when grown in the presence of different electron donors in the laboratory. However, SRB isolated from the tailings were able to grow with low concentrations of organic carbon, formate and acetate detected at concentrations lower than 1 mM in the pore waters appeared to be the main electron donors for SRB. These organic acids likely originated as small metabolic excretion products of living biomass or from the degradation of dead biomass (e.g. Fe-oxidizing bacteria) present in the tailings. SRB locally affected the cycling of Fe in the tailings by promoting the formation of small amounts of Fe-monosulfides. However, the cycling of Fe in the lower portion of the tailings was mainly driven by the precipitation of melanterite (FeSO₄.7H₂O) following the oxidation of pyrite and the release of large amounts of dissolved Fe and SO₄²⁻.