Bacterial biofilms on gold grains-implications for geomicrobial transformations of gold

The biogeochemical cycling of gold (Au), i.e. its solubilization, transport and re-precipitation, leading to the (trans)formation of Au grains and nuggets has been demonstrated under a range of environmental conditions. Biogenic (trans)formations of Au grains are driven by (geo)biochemical processes mediated by distinct biofilm consortia living on these grains. This review summarizes the current knowledge concerning the composition and functional capabilities of Au-grain communities, and identifies contributions of key-species involved in Au-cycling. To date, community data are available from grains collected at 10 sites in Australia, New Zealand and South America. The majority of detected operational taxonomic units detected belong to the α-, β- and γ-Proteobacteria and the Actinobacteria. A range of organisms appears to contribute predominantly to biofilm establishment and nutrient cycling, some affect the mobilization of Au via excretion of Au-complexing ligands, e.g. organic acids, thiosulfate and cyanide, while a range of resident Proteobacteria, especially Cupriavidus metallidurans and Delftia acidovorans, have developed Au-specific biochemical responses to deal with Au-toxicity and reductively precipitate mobile Au-complexes. This leads to the biomineralization of secondary Au and drives the environmental cycle of Au.