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TINY flakes of gold have been 鈥済rown鈥� in the laboratory by microbes from gold-bearing soil. Their ability to concentrate traces of gold could help explain why flakes and nuggets of gold are often found far from a seam, and could lead to improved prospecting techniques, say Australian researchers.

Frank Reith of the Australian National University in Canberra collected soil and gold samples from the Tomakin mine in New South Wales. In the mine itself, gold is very finely dispersed through the quartz rock, but if you pan for gold in the surrounding area, you find whole flakes. 鈥淭he question is, how did they get there?鈥� says Reith.

Researchers already know that some bacteria can release elemental gold from rock. And geologists have reported bubbly formations on the surface of some gold flakes, and suggested these might be due to microorganisms. So when Reith found these formations on some of the Tomakin flakes, he took a closer look. Electron microscopy revealed what look like cell walls, and tests for DNA came back positive: good evidence, Reith says, that the formations really are the remnants of microbial colonies.

Reith then grew fungi and bacteria from soil samples from the mine in solutions containing dissolved gold. Gold crystals began to form, with the gold building up in layers, atom by atom 鈥� the start of a nugget, he says.

There are other chemical and physical processes that might form nuggets, but this new work suggests that microbial action is important too. Understanding all the different mechanisms is critical for gold exploration, says Ernst Kohler, a gold specialist at CSIRO Exploration and Mining in Western Australia, an offshoot of the science research agency.

Reith plans to find out which species of microbe are responsible for forming the gold crystals. Gold prospectors could then concentrate on topsoil that tests positive for these species, he says. Mining companies could also use the microbes to extract and concentrate gold from ores that are currently difficult to dissolve.

But that process could be a slow one. The largest nuggets ever found, including some weighing several kilograms from Victoria, Australia, probably took millions of years to form, says Reith.