‘Gold Bug’ Sheds Light on How Some Gold Deposits Formed

Not
alchemy but microbe droppings, suspects noted UMass microbiologist

For centuries, scientists have
wondered why gold is found in two forms – as a solid in deposits
close to the Earth’s crust, and in solution, often far removed
from gold-ore deposits. A fairly simple lab experiment conducted
at the University of Massachusetts may lead to an understanding
of how the precious metal came to be available in disparate forms,
and how some gold-ore deposits might have been formed.

In research related to pollution clean-up, a team of UMass microbiologists
led by noted researcher Derek Lovley has extracted gold solids
from water containing dissolved gold. The work uses technology
Lovley developed 10 years ago to clean up heavily polluted water
and soil around the U.S. using bacteria and archaea, or ancient
micro-organisms, to break down heavy metals in affected environments.

Like
uranium, cadmium, and other heavy metals, gold is precious and
useful to humans. Lovley notes that dissolved gold, however, is
useless because it can’t be manipulated and formed into objects
of value or beauty. He says when either solid or liquid gold is
ingested, it is toxic to most life forms. On the other hand, liquid
gold and many other heavy metals are not toxic to a group of microbes
called extremophiles, or simple life forms known to thrive in
environments where others cannot live.

With
this in mind, the UMass researchers asked if extremophiles might
have ingested the liquid gold found in hydrothermal vents, hot
springs, and other hot places, and left it scattered as deposits
of solid gold in places that now are below the surface of the
Earth. This would explain how the metal came to be in two different
forms in very different environments. If that is the case, the
team wondered if microbes could duplicate the process in the laboratory
and extract valuable solids from liquid containing dissolved gold.

“A vast number of bacteria and archaea have the ability to transfer
electrons to iron through a reduction process,” explained Lovley.
“In other words, they digest one form of a metal and excrete it
as another form. This transfer leaves behind deposits of solid
metal in unlikely places on Earth or maybe even on Mars. What’s
left behind is often more useful, or more accessible to humans,
than the original form of the same substance.”

Lovley’s
lab has previously published evidence that iron-reducing micro-organisms
are involved in the formation of uranium ores, changing uranium
to a form that precipitates out of water. Massive accumulations
of magnetite created by iron-reducing microbes during the Precambrian
period of the earth’s development now are important deposits of
iron ore, according to Lovley.

In
the laboratory, postdoctoral research associate Kazem Kashefi,
and graduate students Jason M. Tor, and Kelly P. Nevin studied
dissolved gold in an oxidized form in an environment similar to
that found in a hydrothermal vent, where dissolved gold can sometimes
be found.

The
team wanted to see what would happen if they put iron-reducing
microbes into the gold solution under those conditions. As they
suspected, the microbes rapidly converted the gold from the useless,
oxidized, dissolved form to a more valuable, insoluble, metal
form. Essentially, the microbes had eaten the solution, and left
behind a precious by-product.

“There’s a significant amount of gold found in solution in some
thermal springs, and hydrothermal vents on the ocean floor,” Lovley
said. “The problem is that the gold is extremely diluted, so only
a teeny amount is dispersed in a very large volume of water.”

“There are waste streams from gold processing where this same
reduction process might work on a larger scale, but the goal of
this study was to offer an explanation of how gold deposits are
formed, more than it was to produce any profitable or useful application
on a larger scale,” explained Lovley. The research was presented
in the July issue of the journal Applied and Environmental Microbiology.
It was funded in part by a grant from the National Science Foundation,
through the Life in Extreme Environments Program.

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Derek
Lovley can be reached at 413-545-9651 or dlovley@microbio.umass.edu.