Bacteria Make Gold Nuggets

The finding could help prospectors use biosensors to zero-in on where clumps of the precious metal may lie.

THE GIST

- For the first time microbes have been shown to tinker with gold deposits.

- Bacterial biofilms dissolve gold, which makes it available for re-depositing in purer form elsewhere.

- The bacteria have been identified and so someday hand-held biosensors may help in gold prospecting.

Gold nuggets are often the creations of bacterial biofilms, say Australian researchers who have demonstrated the process and even identified the bacteria at work.

Layers of bacteria can actually dissolve gold into nanoparticles, which move through rocks and soils, and then deposit it in other places, sometimes creating purer "secondary" gold deposits in cracks and crevices of rocks. The process overturns the long-held belief by some scientists that gold ore is created only by "primary" physical geological processes.

By looking at the DNA in biofilms that grow on gold grains collected from the Prophet gold mine in southeast Queensland, Australia , the University of Adelaide's Frank Reith and his colleagues discovered that 90 percent of the bacteria were of just two species Delftia acidovorans and Cupriavidus metallidurans. The bacteria share genes that make them resistant to the toxic effects of heavy metals.

"It's the first time we actually see the mechanism laying on top of the gold grain," said Joël Brugger of the South Australian Museum and University of Adelaide, a co-author on a report about the discovery which appears in the September issue of the journal Geology.

"We tagged the DNA and saw the beautiful active biofilm (dissolving the gold)," said Brugger. "That was very interesting because gold in soluble form is very toxic." That dissolved gold can then be redeposited in other places in a much purer form.

The discovery is especially important because it could point to a new high tech way to prospect for gold.

"They have been looking for gold in Australia for a hundred years," said Brugger. "It's getting more and more difficult." In fact most of the gold mining activity is just extensions of old discoveries made decades ago. "Finding something new is really, really rare."

One thing that makes it particularly hard to find new gold deposits in southeast Queensland is that the rocks there, and over most of Australia, are some of the oldest on Earth and have been largely ground down and buried by many meters of soil.

Finding gold deposits has often meant chancing upon the element on or near the surface soil, then digging down in search of ore-bearing rocks. Sometimes that has worked, sometimes not, said Brugger.

"At the moment we don't really understand how gold moves around in the environment," said Brugger. "I think that here we can see for the first time how it happens." Microbes dissolve it and move it around with the groundwater flow -- which can be pretty quick.

The presence of the bacteria could be a quick way to test if gold is present in the ground, Brugger suggested. Field geologists could even someday use biosensors that are tuned to detect the genes of these gold-specific microbes.

"It may have a direct application in understanding how gold is going to exist in these environments," agreed geochemist D.C. "Bear" McPhail of Australian National University. McPhail is looking at how microbes might alter the concentrations of different uranium isotopes, also a toxic metal, in soils, which can effect radioisotope dating techniques.

"We still have some way to go until we have direct application," said McPhail. "But it may lead to looking in different parts of the soil. It gets more and more sophisticated all the time."

Many gold nuggets are formed after layers of bacteria dissolve gold into nanoparticles, which move through rocks and soils before forming clumps.

The Invisible Squid

The Hawaiian Bobtail Squid, Euprymna scolopes, has a clever way of duping predators during its nightly activities. It uses a symbiotic luminescent bacteria, Vibrio fischeri, to light up its underside, so that upwards-looking predators don't see a dark, edible form silhouetted against a moonlit or starlit sky. Instead, hungry sharks or other fish see only sky. The squid is invisible. And while this is very cool, there's a whole lot more to this symbiosis story.

Lighting Up

What the glowing bacteria get out of this arrangement is a comfy place to live, food and even help reproducing. But because the squid does not need the bacteria except as a camouflage, it can live happily without it in a laboratory. "The squids are not physically compromised without the bacteria," said University of Wisconsin biologist Margaret J. McFall-Ngai, who has studied the squid and its symbiotic bacteria for two decades. This makes it possible to look at the relationship of the two organisms in great detail. The same cannot be said of a human's gut bacteria, she said. They are not so good at living apart, and there are so many kinds of them. The one squid, one bacteria model simplifies the study tremendously, she said.

Culturing Bacteria

Among the things that have been discovered by studying this relationship is that the bacteria and the squid operate together on a daily rhythm, said biologist Spencer Nyholm of the University of Connecticut. Every morning the squid spits out 95 percent of the glowing bacteria, along with some of its own cells, perhaps to feed the bacteria. These expelled bacteria are then taken in and grown by other young squids. After expelling the bacteria, the squid buries itself in sand and rests for the day, growing a new batch of glowing bacteria, which only glow when they reach a certain concentration.

Cross-Species Communication?

How do the bacteria "know" when they reach that concentration? That's another important mystery that needs to be found out. "You see similar things in the guts of humans and other animals," said Nyholm, the second author on a paper on a recent issue of the Proceedings of the National Academy of Sciences reporting on the biochemical mechanisms behind the daily rhythms in both organisms.

What About Us?

Understanding the inner workings of these rhythms could lead to new ways to treat disease. Among the big questions that the squid and bacteria could answer, for instance, is how the two organisms communicate so they don't harm each other. "I'm interested in specifically how the immune system reacts," said Nyholm. "How can they tell the good from the bad bacteria?" Indeed, how does the human body know the good gut bacteria from the bad bacteria? And why don't our gut bacteria just keep growing and kill us? "That doesn't happen (in the squid) and that doesn't happen in us either," said McFall-Ngai. Discovering why could, among other things, lead to new ways to fight bacterial infections, since the molecules involved in the process are, remarkably, the same in squids and humans.

Symbiosis

The very fact that the molecules at work are the same in such very different animals suggests something else too, she said. Unraveling the fine details of this symbiotic relationship could open a window into some very fundamental and ancient processes that date back to the earliest life on Earth. That's some big science from a small glowing squid.