Biogeochemical cycling of gold: Exploring the links between gold transformation, microbial communities, biogeochemical processes and mineralisation style

Abstract

The biogeochemical cycle of gold (Au), wherein biofilms contribute to the transformation of Au particles, has been demonstrated from various ecological and environmental settings. In this study, we explore the links between Au particle transformation and (bio)geochemical processes including deposit style/regional mineralisation, microbial community composition, position in landscape and physical weathering. A total of 114 placer Au particles were characterised using scanning electron microscopy collected from the UK (eight sites), Switzerland (eight sites) and Germany (14 sites). These placer Au particles demonstrated different degrees of alteration as shown by a range of morphology and surface textures. Collectively, these Au particles represented a continuum of Au particle transformation mediated by microorganisms and geochemical and physical processes. Additional evidence of Au biogeochemical cycling at the interface of the Au particle surface was demonstrated by the abundance of polymorphic layers containing clay minerals in which microbial cells, extracellular polymeric substances (EPS) and Au nano- and micro-particles were closely associated. A total of 260 placer Au particles were assessed for resident biofilm community composition using nested 16S rRNA polymerase chain reaction (PCR) combined with next generation sequencing (NGS) using the Illumina MiSeq platform. Findings from UK Au particles demonstrated that the biofilm community assemblages evolved to a more similar composition of metal-resistant members as Au particles progressively became more transformed. Analysis of 36 placer Au particles from Switzerland suggested that transformation stages of Au were highly influenced by the landscape setting leading to the recruitment of diverse members of the biofilm community with increased distance of Au particle transport. A total of 198 placer Au particles from Germany were quantitatively mapped using the Cameca™ SXFive Electron Microprobe. The analysis demonstrated that the conditions of epithermal and hydrothermal deposits produce the difference in Au:Ag ratios that ultimately dictate how the microbial communities sense the difference in the Au-Ag ‘fabric’ and therefore direct Au transformation differently. Proteobacteria dominated the community assemblage from all sites in UK, Switzerland and Germany (>30 % of total OTUs and sequencing reads) suggesting that highly-specialised biofilm communities adapted to Au-toxicity perform key roles in the transformation of Au particles. These include the genera Pseudomonas, Leptothrix and Acinetobacter detected in Germany placer Au in addition to putative exoelectrogenic genera Rhodoferax and Geobacter that were highly abundant on Au particles from the UK. This study also reports the contribution of biofilm communities to the mobility of Au in iron (Fe)-, organic matter (OM)- and calcium (Ca)-rich soil. Soil columns with Au particles placed in pouches and inoculated with active Au mobilisers Chromobacterium violaceum and Corynebacterium glutamicum, and precipitating organism Cupriavidus metallidurans demonstrated that Au was mobilised either as a complex (up to 320 mg/L Au) or natural Au nanoparticles (AuNPs) (~27–38 nm), respectively. These suggest amino acid- and cyanide-complexation or reductive precipitation in the periplasm leading to Au mobility. The results of this study will improve the use of (bio)geochemical models to predict Au transport for applications in Au exploration and to advance the understanding of the biogeochemical cycle of Au in other environmental and climatic regions across the globe.