Effect of physical and biogeochemical factors on placer gold transformation in mountainous landscapes of Switzerland

In surface environments, placer gold particles are (re)shaped by physical, chemical and biogeochemical processes. This study hypothesizes that in modern landscapes combinations of physical and biogeochemical factors determine placer gold transformation and the composition of biofilm communities living on particle surfaces. Morphology, composition and biofilm communities of placer particles from eight locations in Switzerland were analyzed. Sites reflect a variety of modern-day placer environments in mountainous terrains, ranging from eluvial sites in close proximity to primary sources, to alluvial sites associated with both slow- and fast-moving streams, and to distal glacio-fluvial and depositional lake sediments. Particles are progressively physically and biogeochemically transformed with increasing distance from source. Variable degrees of rounding, gouging and scratching from physical reshaping due to transport and sedimentation processes are evident especially on particles from high energy environments. Surface textures indicative of biogeochemical gold/silver dissolution are common. In low energy/depositional environments and with increasing distance from source overgrowths of gold nano- and micro-particles and aggregates are abundant. These form micro-crystalline layers of secondary gold on particle surfaces covering them to depths of tens of micrometers. Biofilm communities across all sites are dominated by Proteobacteria (669 Operational Taxonomic Units (OTUs), 49.8%), especially β‑Proteobacteria. Other important phyla include Acidobacteria, (172 OTUs, 9.7%), Bacteroidetes (222 OTUs, 11.6%), Cyanobacteria (26 OTUs, 13.7%), and Planctomycetes (125 OTUs, 7.1%). Canonical analysis of principal coordinates demonstrates significant links of community assemblages to biogeochemical transformation and landscape position of collected particles. Increasing specialization towards metallophilic phyla was recorded on particles collected with increasing distance from source, suggesting that Au toxicity resulting from progressing biogeochemical transformation affected bacterial communities. This study ultimately shows that placer gold transformation and biofilm composition within modern landscapes is highly dependent on the interplay of physical and biogeochemical factors.