Re: bacterial precipitation


chriso wrote:
Bacteria linked to gold nugget formation
By Stephen Pin*** for Science Online

Australian scientists have found the strongest evidence so far that
bacteria play a key role in forming gold grains and nuggets.

They have found bacteria that remove gold from the soil and deposit pure
grains of it around them.

Researcher Dr Frank Reith, from the CRC for Landscape Environments and
Mineral Exploration, and colleagues gathered their evidence at two
separate mines and publish their results today in the journal Science.

At the Tomakin mine on the south coast of New South Wales and the Hit or
Miss mine in tropical north Queensland, most gold is hidden away in
quartz veins, in amounts that are invisible even to high-powered
microscopes.

But the soil above the mines also contains grains and nuggets of gold
that have somehow found their way out of the quartz.

"There are a probably a lot of processes involved," Dr Reith said.

He and other scientists have long suspected that bacteria play a part,
but it is an idea that has generated some scepticism.

To test the theory, Dr Reith sifted the soil above the mines and
collected gold grains 0.1-2.5 millimetres across, and then subjected them
to several experiments.

Bubbles of gold

First, Dr Reith looked at the grains under a high-powered electron
microscope to confirm that they contained bacteria-shaped bubbles of
gold.

"They're little lumps on the surface," Dr Reith said.

Next, he looked for organic matter on the grains, as evidence that
bacteria had been growing on their surfaces.

Finally, he used a technique called polymerase chain reaction to look for
bacterial DNA on the surfaces of the grains to show that living bacteria
are still there.

"The DNA would have degraded if the bacteria weren't around any more," Dr
Reith said.

Dr Reith says about 80 per cent of the grains had living bacteria on them
and the only bug that was found on all those positive samples was
Ralstonia metallidurans.

"These grains come from areas that are almost at the opposite ends of
Australia," he said.

"We were pretty happy with that."

Toxic gold

Dr Reith thinks the Ralstonia bacteria play an important role in the
microbial ecosystem in soil, helping to rid it of the soluble gold that
most other species find toxic.

"This is the guy whose job it is to get the toxic gold out of the
environment so the other bacteria can live a happy life," he said.

In the future, the gold-loving bugs could prove a boon to industry, Dr
Reith says.

They could perhaps be used to improve gold processing, or even be useful
as a marker for the presence of gold that is otherwise invisible.

Is this new? Some years ago I went around the tourist route in the
Dolaucothi gold mines in Wales. The guide pointed some glittering
trails on the walls and explained that they were the result of
bacterial action slowly concentrating the gold. The speed of the
process could be guaged from the time since that part of the mine had
last been worked, and we were told that it was far too slow to be of
commercial use.

I tend to be sceptical about the knowledge of tour guides - they don't
usually let reality get in the way of an exciting yarn - but this
sticks in my mind.

Robin.

p.s. If I were the author of this article, I'd get my name changed :-)

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