Anthropogenic mercury accelerates natural gold biogeochemical cycling

On Earth’s surface environment, gold is progressively transformed through various biochemical and geochemical processes. Previous studies have primarily addressed gold biogeochemical cycling in relatively pristine environments; however, small-scale and illegal mining practices still use mercury for gold extraction. Therefore, this study aims to assess the effect of anthropogenic mercury on natural gold biogeochemical cycling. The presence of mercury in gold-bearing soils was interpreted to be the primary factor that alters natural gold particle morphology as well as surface textures, i.e., secondary gold structures. Additionally, the toxicity of mercury contemporaneously exerted a selective pressure on the microbial communities on gold particles. The enrichment and selective enrichment of bacterial communities from gold particles from mercury-contaminated and non-contaminated sites establish the link between the effect of long-term mercury and enrichment of heavy metal resistant/transforming bacterial communities. Whole genome sequence analysis of two bacteria from mercury-contaminated sites placer gold particles demonstrates the presence of diverse heavy metal resistant genes including mercury resistant mer operons. This suggests the underlying mechanisms of bacterial communities resistance to highly toxic soluble Au (I/III) complexes as well as the effect of long term heavy metal including mercury stress. This study finds a natural geochemical affinity of mercury with gold and its toxicity which enrich metallophilic bacteria on placer gold particles, together catalyse a faster biogeochemical transformation of gold. Overall results indicate that the biogeochemical cycle of gold is accelerated 3 fold than the natural rate in the mercury affected placer gold particles.