The smallest gold-diggers in the world
Bacteria found in Australian mines help gold grains to form.
Prospectors looking for gold nuggets have swarms of tiny helpers: bugs that take up toxic gold complexes from the soil and spit out pure gold on to the grains around them. Research published today provides strong evidence that bacteria known to produce gold in the laboratory do their trick in the wild too.
Impure gold, mixed with silver and copper, is found in the veins of minerals such as quartz, sometimes in such low concentrations as to be invisible. More obvious to the casual prospector are the highly enriched 'secondary grains' or nuggets found in river beds and soil. It is thought that these may be formed by weathering processes that either leach out the silver and copper metals, or wash the gold into solution so it can accumulate elsewhere.
Frank Reith of the Australian National University in Canberra and his colleagues have shown that microbes play a role too.
Geologists in the field have seen bacterial structures on gold grains before, and microbiologists in the laboratory have shown that many types of bacteria will deposit gold and other metals from solution. But the link between bacteria and gold deposits was tenuous. Now Reith and his team have bridged the divide.
The researchers visited gold mines 3,400 kilometres apart, in New South Wales and Queensland, Australia. "We shovelled up gold into pans, and looked at the gold flakes. Forms like cell walls appeared bacterial remnants encased in gold," says Reith.
Genetic analysis revealed up to 30 species of microbe in the biofilms of bacteria on the gold grains. One that was found on gold grains in both sites but never in the surrounding soil was identified as the bacterium Ralstonia metallidurans, a microbe that can grow in metal-rich solutions.
The team, whose results are reported in Science1, showed that R. metallidurans could survive in solutions of toxic gold chloride, and precipitated gold as it did so.
"Ralstonia may be precipitating the gold to detoxify its environment meaning that it and the other organisms in the biofilm can survive," says Reith. "It's possible that the bacteria become encased in gold and other bacteria grow on top, like a coral reef."
The exact details of how or why microbes enrich gold nuggets are still unclear.
"Gold chloride complexes are rare in surface environments," comments Gordon Southam, a geomicrobiologist from the University of Western Ontario, Canada. He thinks that thiosulphate gold complexes may also be involved but this study didn't look to see how the bugs react to that chemical brew.
"We need to try putting mobile gold into the soil and adding organisms to that," says Reith. "Could we then find gold nuggets? Would this take months, years, tens of years?"
Potential applications from sensing the bacteria as a way to look for gold mines, to using them to help make industrially useful particles of gold may be some way off. But Southam is keen to exploit the bacteria's natural trick. "I want to make a gold nugget one day," he says. "Gold nuggets grow in nature, so why shouldn't I be able to make one?"
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- Reith F., et al. Science, 313. 233 - 236 (2006).