Effect of resident microbiota on the solubilization of gold in soil from the Tomakin Park Gold Mine, New South Wales, Australia

The processes influencing the solubilization and observed mobility of Au in soil were studied using a combination of geochemical and microbiological techniques. In this study, we demonstrate for the first time that biotic processes mediated by the resident microbiota are likely to control the mobilization of Au in auriferous soils and other regolith materials. Microcosms with auriferous soils from the Tomakin Park Gold Mine in temperate south eastern New South Wales, Australia, were incubated under biologically active versus inactive (sterilized) conditions. The soils were incubated oxic and anoxic, unamended and Au pellet- or cycloheximide amended for 70 days in a 1:4 (w:v) aqueous slurry at 25 °C in the dark. In biologically active unamended Ah- and B-horizon microcosms up to 80 wt.% of total Au was detected in solution after 45 days of incubation. In biologically active Au pellet amended microcosms Au was liberated from the soil and also from added Au pellets. Scanning electron microscopy and nucleic acid staining combined with confocal stereo laser microscopy revealed the presence of bacterial biofilms on Au pellets incubated in the biologically active microcosms. The biologically inactive microcosms displayed no or significantly reduced Au solubilization. After 40-50 days of incubation Au was generally re-adsorbed to the solid soil fractions. The results of sequential extractions conducted with dried slurry samples collected from the biologically active Ah-horizon microcosms after 0, 10, 20, 30, 40, and 68 days of incubation indicated a continuous microscale solubilization and re-adsorption of Au. In samples taken after 40 days of incubation more than 80 wt.% of the Au was extracted from the operationally defined organic fraction, which appears to act as a final re-adsorption site for Au in the soil. In samples taken after 10 days of incubation from microcosms amended with 100 μg g⁻¹ (d.w. soil) of Au as AuCl₄⁻ 95 wt.% of the Au was associated with the organic fraction. To establish a mechanistic link between Au dissolution and re-adsorption with the activity of the heterotrophic bacterial community, analysis of the community structure based on carbon utilization patterns using was conducted. The bacterial community structure changed from a carbohydrate- and polymer-utilizing to a carboxylic- and amino acid utilizing community concurrently with the change from Au solubilization to re-adsorption. The bacterial community in the early stages of incubation (0-30 days) apparently produced an excess of amino acids, which are known to form stable amino acid Au complexes. The bacterial community in the later stages of incubation (after 40-50 days) metabolized these Au complexing ligands and Au, which apparently became unstable in the solution, was re-adsorbed to the solid soil fractions.