Co-operative leaching and chemotaxis of thiobacilli studied with spherical sulphur/sulphide substrates

The Relationship between Acid Production and the Microbial Community of Newly Produced Coal Gangue in the Early Oxidation Stage

Coal gangue is a solid waste formed during coal production, and the acid mine drainage it generates during open-pit storage severely pollutes the ecological environment of mining areas. Microorganisms play a crucial catalytic role in acidification, and their species and gene functions change during the oxidation process of coal gangue. In this study, the changes in microbial community structure were investigated during the initial acidification process for newly produced gangue exposed to moisture by monitoring the changes in pH, EC, sulfate ion concentration, and the iron oxidation rate of gangue leaching solutions. Moreover, the composition and functional abundance of microbial communities on the surface of the gangue were analyzed with rainfall simulation experiments and 16S rRNA sequencing. The study yielded the following findings: (1) The critical period for newly produced gangue oxidation spanned from 0~15 d after its exposure to water; the pH of leaching solutions decreased from 4.65 to 4.09 during this time, and the concentration and oxidation rate of iron in the leaching solutions remained at low levels, indicating that iron oxidation was not the main driver for acidification during this stage. (2) When the gangue was kept dry, Burkholderia spp. dominated the gangue microbial community. When the gangue was exposed to moisture, the rate of acidification accelerated, and Pseudomonas replaced Burkholderia as the dominant genus in the community. (3) In terms of gene function, the microbial community of the acidified gangue had stronger nitrogen cycling functions, and an increase in the abundance of microorganisms related to the sulfur cycle occurred after day 15 of the experiment. The microbial community in the acidified gangue had more stress resistance than the community of the newly formed gangue, but its potential to decompose environmental pollutants decreased.

Investigation and Visualisation of Microbial Attachment Trends to Sulphide Minerals in a Bioleach Environment

Two novel experimental approaches have been developed to investigate the attachment of microorganisms to sulphide minerals as a function of the mineral and microbial phase in a joint project with BHP Billiton, conceptualised in 2005 and initiated in 2006. In the first approach, selective attachment of pure cultures to minerals was studied in the “particle coated column reactor” using A. ferrooxidans and L. ferriphilum. The saturated reactor containing glass beads coated with fine mineral concentrates provided a quantifiable surface area of mineral concentrate and maintained good fluid flow. Results are reported for chalcopyrite and pyrite concentrates, a low grade chalcopyrite ore and quartzite. The latter, representing typical gangue material, is used as a control. A. ferrooxidans displayed greater attachment to pyrite, and selective attachment to sulfide minerals over quartz. Similar attachment behaviour trends resulted for Leptospirillum spp. In the second approach, a novel technique was developed to investigate microbial ecology of microbe-mineral attachment, site and mineral specific associations of microorganisms and spatial organisation of microbial communities present. Qualitative assessment and visualisation of microorganisms associated with the mineral surface and subsequent biofilm development was shown in the biofilm reactor, using microscopy techniques and fluorochromes. FISH analyses of A. ferrooxidans and L. ferriphilum on massive chalcopyrite sections are presented. The consequence of the observed attachment on heap bioleach performance is discussed.

Molecular cloning, sequencing, and expression of omp-40, the gene coding for the major outer membrane protein from the acidophilic bacterium Thiobacillus ferrooxidans

Thiobacillus ferrooxidans is one of the chemolithoautotrophic bacteria important in industrial biomining operations. Some of the surface components of this microorganism are probably involved in adaptation to their acidic environment and in bacterium-mineral interactions. We have isolated and characterized omp40, the gene coding for the major outer membrane protein from T. ferrooxidans. The deduced amino acid sequence of the Omp40 protein has 382 amino acids and a calculated molecular weight of 40,095.7. Omp40 forms an oligomeric structure of about 120 kDa that dissociates into the monomer (40 kDa) by heating in the presence of sodium dodecyl sulfate. The degree of identity of Omp40 amino acid sequence to porins from enterobacteria was only 22%. Nevertheless, multiple alignments of this sequence with those from several OmpC porins showed several important features conserved in the T. ferrooxidans surface protein, such as the approximate locations of 16 transmembrane beta strands, eight loops, including a large external L3 loop, and eight turns which allowed us to propose a putative 16-stranded beta-barrel porin structure for the protein. These results together with the previously known capacity of Omp40 to form ion channels in planar lipid bilayers strongly support its role as a porin in this chemolithoautotrophic acidophilic microorganism. Some characteristics of the Omp40 protein, such as the presence of a putative L3 loop with an estimated isoelectric point of 7.21 allow us to speculate that this can be the result of an adaptation of the acidophilic T. ferrooxidans to prevent free movement of protons across its outer membrane.