Abstract
Soluble arsenic (As) in acidic feed solution may inhibit the copper (Cu) bioleaching process within mine heaps. To clarify the effect of soluble arsenic on the live biomass and bioxidative activity in heaps, toxicological assays were performed using a synthetic feed solution given by a mine company. The microorganisms had previously been isolated from two heap samples at up to 66 m depth, and cultured using specific media for chemolithotrophic acidophiles (pH 1–2) and moderate thermophiles (48°C), for arsenic tolerance assay. The four media with the highest biomass were selected to assay As-resistance; one culture (Q63h) was chosen to assay biooxidative activity, using a heap sample that contained chalcopyrite and covellite. We found that 0.5 g/L of As does not affect living biomass or biooxidative activity on Cu sulfides, but it dissolves Cu, while As precipitates as arsenic acid (H3AsO4·½H2O). The arsenic tolerant community, as identified by 16S rDNA gene sequence analysis, was composed of three main metabolic groups: chemolithotrophs (Leptospirillum, Sulfobacillus); chemolithoheterotrophs and organoheterotrophs as Acidovorax temperans, Pseudomonas alcaligenes, P. mendocina and Sphingomonas spp. Leptospirillum spp. and S.thermosulfidooxidans were the dominant taxa in the Q63–66 cultures from the deepest sample of the oldest, highest-temperature heap. The results indicated arsenic resistance in the microbial community, therefore specific primers were used to amplify ars (arsenic resistance system), aio (arsenite oxidase), or arr (arsenate respiratory reduction) genes from total sample DNA. Presence of arsB genes in S. thermosulfidooxidans in the Q63–66 cultures permits H3AsO4-As(V) detoxification and strengthens the community’s response to As.
Highlights
A mine heap is an imposing man-made “deposit” where crushed ore and low-grade minerals are agglomerated and stacked to a height of up to 100 m and hundreds of meters in length and width
Because our results indicated the presence of As resistance, putatively because of the presence of ars, aio, or arr, we searched for the presence of aio, arsB, and arr3 in organisms in the enriched cultures
According to the growth curves calculated for each selected culture, maximum biomass was reached at day 4 (Fig. 1)
Summary
A mine heap is an imposing man-made “deposit” where crushed ore and low-grade minerals are agglomerated and stacked to a height of up to 100 m and hundreds of meters in length and width. Within such mine heaps, bioleaching processes occur. The commonly used process of bioleaching is feeding a heap with an acidic (pH < 2) “leach solution”, to promote chemolithotrophic activity of thermoacidophilic microorganisms living in the heap; the microorganisms catalyze the solubilization or bioleaching of economically valuable metals, as. Bioleaching in mine heaps is a widely-employed process used to extract metals from low-grade ores, which would not be economically extracted by any other method. Because of the huge spatial–temporal heterogeneity of mine heaps in terms of characteristics as mineral reactivity, irrigation efficiency, temperature, pH, and partial pressure of O2, CO2, redox potential, dissolved solutes, and available nutrients, considerable diversity of resident acidophilic Archaea and Bacteria is expected, including
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