Effect of the cyanide-producing bacterium Chromobacterium violaceum on ultraflat Au surfaces

Solubilization, transport and re-precipitation of Au in the supergene environment can lead to the formation of secondary Au enrichment zones, revealed as geochemical anomalies and secondary gold grains. Cyanide-producing microorganisms can contribute to the solubilization of Au, and the cyanide produced may play an important role in the formation of ‘bacterioform’ structures observed on gold grains. To examine the effect of bacterial cyanide production on Au surfaces, ultra-flat Au foil was incubated in peptone meat extract (PME) medium in the presence of the cyanide-producing bacterium Chromobacterium violaceum for up to 56 days. Within seven days C. violaceum had formed biofilms on the Au surfaces, and cyanide produced by the bacteria had a measurable effect on the Au surface compared to abiotic controls. After removal of the biofilm by sonication in de-ionized water, the surfaces incubated with C. violaceum were significantly rougher compared to the negative controls. This was demonstrated by root mean square roughness (RMS) analyses and statistical t-testing following scanning tunneling microscopy (STM). Analyses of Au and cyanide in the growth medium supported the STM results: After 56 days of incubation 125 parts-per-billion (ppb) of Au was detected in solution or associated with C. violaceum cells. In contrast, Au solubilization and cyanide formation was not observed in negative controls. These experiments provide evidence that bacteria produce metabolites capable of mediating Au solubilization that may lead to the dispersion of Au in the environment, and suggest that dissolution of Au may contribute to the formation of ‘bacterioform’ structures commonly observed on secondary gold grains.