Metal resistant bacteria on gold particles: Implications of how anthropogenic contaminants could affect natural gold biogeochemical cycling

In Earth's near-surface environments, gold biogeochemical cycling involves gold dissolution and precipitation processes, which are partly attributed to bacteria. These biogeochemical processes as well as abrasion (via physical transport) are known to act upon gold particles, thereby resulting in particle transformation including the development of pure secondary gold and altered morphology, respectively. While previous studies have inferred gold biogeochemical cycling from gold particles obtained from natural environments, little is known about how metal contamination in an environment could impact this cycle. Therefore, this study aims to infer how potentially toxic metal contaminants could affect the structure and chemistry of gold particles and therefore the biogeochemical cycling of gold. In doing so, river sediments and gold particles from the De Kaap Valley, South Africa, were analysed using both microanalytical and molecular techniques. Of the metal contaminants detected in the sediment, mercury can chemically interact with gold particles thereby directly altering particle morphology and “erasing” textural evidence indicative of particle transformation. Other metal contaminants (including mercury) indirectly affect gold cycling by exerting a selective pressure on bacteria living on the surface of gold particles. Particles harbouring gold-tolerant bacteria with diverse metal resistant genes, such as Arthrobacter sp. and Pseudomonas sp., contained nearly two times more secondary gold relative to particles harbouring bacteria with less gold-tolerance. In conclusion, metal contaminants can have a direct or indirect effect on gold biogeochemical cycling in natural environments impacted by anthropogenic activity.