Optimizing the detachment of ore agglomerate bound microorganisms

Abstract

By bioleaching metals can be economically recovered from low-grade ores and waste ore, which would not be possible traditional mining methods. In bioleaching, dissolution of metals from sulfide ores is catalyzed by chemolithoautotrophic microorganisms that can survive in the extremely acidic mining environment. In heap leaching as much as 99% of microorganisms responsible for bioleaching are attached to the ore surface. However, the microbial abundance and diversity analyses are typically performed from leach liquor samples and, thus may result in limited or even biased view of microbial communities within the heaps. Although various techniques have been used for cell detachment generally optimized methods do not exist. The aim of this study was to optimize a method for the detachment of microorganisms from ore surfaces. Optimization was done using model organisms Acidithiobacillus ferrooxidans, Sulfolobus metallicus and a mesophilic enrichment culture. Microbes for the enrichment originated from heap leach liquor samples from Terrafame mine located in Sotkamo, Finland. The experiments were conducted using agglomerate of polymetallic black schist ore which also originated from the Terrafame mine. Microorganisms were attached onto the ore surface by recirculating culture solution (180 mL) through a column containing 200 g ore agglomerate for 24 h. For detachment, homogenized 15-g subsamples were taken from the columns and subjected to sonication procedures. Bacterial and archaeal cell abundances were determined using quantitative PCR. Cell counts based on DAPI staining and microscopy were also performed, however, the reliability of the microscopy results was compromised by non-cellular particles after detachment procedures. The recovery percentages of microbes were compared using commercial soil DNA ex-traction kit with and without sonication pre-treatment. With At. ferrooxidans, S. metallicus and enrichment culture over 94%, 99% and 95% of the cells were attached, respectively. Using commercial soil DNA extraction kit the percent recoveries of At. ferrooxidans and S. metallicus were 24±9 and 66±65, respectively. For At. ferrooxidans detachment with sonication pre-treatment was always more effective than without sonication pre-treatment. For S. metallicus detachment without sonication pre-treatment was more effective most of the time. Nevertheless, by using the optimized sonication pre-treatment percent recoveries up to 100% were achieved with both model organisms. With mixed culture 3.5±5% of the bacteria and 4.7±4% of the archaea were recovered without the sonication pre-treatment and the sonication did not improve the % recovery. The attachment method used in this work demonstrated the fast attachment of bioleaching microorganisms to agglomerated ore matrix. Archaea attached to the ore agglomerate more effectively than bacteria and to our knowledge this is first time the difference in attachment to the ore surface between bacteria and archaea has been compared quantitatively. This work also demonstrated that by using the developed microorganism detachment method, it is possible to recover microorganisms from the ore agglomerate matrix and that qPCR combined with quantitative DNA retrieval and extraction is a good method for determining microorganism abundance in bioleaching environment. The developed detachment method should be usable with different kinds of minerals.