Gold! The yellow metal has become the driving force of history many times. With a vision of fabulous wealth, famous seafarers set off on long journeys. Hot on their heels were adventurers and cutthroat types Hernán Cortés whose Francisco Pizarro and in their desire for gold they subverted mighty empires. Gold rushes set millions of people in motion in the New World, Africa and Australia.
The golden cycle in the guts of bacteria
Gold deposits were formed under high pressures and temperatures deep below the earth’s surface. Geologists have long believed that tiny flakes of gold are released into nature precisely from these mother veins of gold. Only recently has it become clear that soil bacteria often play an important role in their formation. Some microbes can dissolve gold and release it in the form of ions into the environment. In its soluble form, gold is carried by underground and surface waters and ends up in ocean sediments.
However, soluble gold may not be irretrievably lost. Other bacteria pick up the precious element again and convert it into an insoluble form of the pure metal. Sometimes small particles “extracted” by bacteria can aggregate into larger formations and create nuggets weighing up to several tens of kilograms. The mystery remained how fast such natural recycling of gold takes place and how it is possible that bacteria are not poisoned by highly toxic soluble forms of gold.
A microbe that “feeds” on copper
Examination of particles from Australia’s famous West Coast Creek gold-bearing site has shown that gold returns to an insoluble form relatively quickly. The grains did not form for more than 60 years, and many formed in less than two decades. We also have more clarity in the detoxification mechanisms of “golden” bacteria. A microbe revealed them to scientists Cupriavidus metalliduranswhich thrives in soils with a high content of various heavy metals.
An electron microscope image shows structures formed by bacteria. The grain of gold comes from a quarry called Hit or Miss, located in Queensland, Australia (photo: CSIRO, CC0)
The bacterium lets the toxic soluble gold into its cell along with the copper it absolutely needs to live. However, high concentrations of copper and gold are fatal for it, and in combination, both elements have an even more fatal effect on the bacterial cell than each metal alone. Bacteria can excrete both elements in soluble form, but this detoxification mechanism is paralyzed by the joint effect of copper and gold. That’s why Cupriavidus metallidurans it prevents the excessive intake of these elements and already on its surface changes the soluble forms of copper and gold again into particles of insoluble pure metals.
Scientists want to use the revealed details of the gold cycle in nature to develop new methods for recycling gold from a variety of wastes, including discarded electronics, from which gold recovery is not yet economically viable.
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The relatively quick process also opens completely new possibilities for mining gold from rocks where the content of this element is low, or from previously mined tailings. In contrast to industrial mining methods, which use, for example, the dissolution of gold with highly toxic mercury, a biotechnological method with the deployment of bacteria would be significantly more environmentally friendly.