Bacteria worth their weight in gold

GOLD nugget-forming bacteria could optimise extraction processes from ore, reprocessing of tailings and recycling from electronic components.

Recent research shows that microorganisms can dissolve and re-concentrate gold on timescales as short as years.

You may have heard of the carbon cycle before, where a single carbon molecule can pass from the atmosphere on to living things and even the deep ocean. This turnover of substances is one of many biogeochemical cycles that exist, which are often driven by microbial action. Microbes, such as bacteria and archaea, are incredibly adaptable and can alter the geochemical conditions of their surrounding environment. This can be on both the micrometre and global scale, through a range of biochemical processes such as oxidation and reduction processes for energy procurement.

Gold is no exception, and recent Australian research has provided valuable insight into the biogeochemical cycle of gold. Surprisingly, it was found that the timescales involved are much shorter than anticipated.

Lead researcher, Gordon Southam from The University of Queensland, said: “In the natural environment, primary gold makes its way into soils, sediments and waterways through biogeochemical weathering and eventually ends up in the ocean. On the way, bacteria can dissolve and re-concentrate gold; a process which removes most of the silver and forms gold nuggets.

“Our results show that this transformation takes place in just years to decades – a blink of an eye in terms of geological time.”

This research, published in Chemical Geology, involved the analysis of several gold grains using high-resolution electron microscopy. Crevices were found on grain surfaces, which were filled with minerals in which gold colloids and platelets were embedded. These ‘secondary’ gold structures pointed to the precipitation of gold by reducing microbes, while further analysis of grains showed that gold-to-silver ratios known to be associated with microbial processes were present.

It was demonstrated that five ‘episodes’ of gold biogeochemical cycling had occurred on each gold grain. Each episode was estimated to take between 3.5–11.7 years, meaning a total of 18–60 years to form the secondary gold.

The paper’s lead author, Jeremiah Shuster from The University of Adelaide, said: “This could help mineral exploration by finding undiscovered gold deposits or developing innovative processing techniques.”

“If we can make this process faster, then the potential for re-processing tailings and improving ore-processing would be game-changing. Initial attempts to speed up these reactions are looking promising.”