No other life form on our planet has infiltrated every environment as
successfully as the minuscule single cells of bacteria. Amongst their many
roles in life on Earth, it turns out some of these microbes are also experts
at purifying precious metals.
An international team of researchers has figured out how one metal-gobbling
bacterium, Cupriavidus metallidurans, manages to ingest toxic metallic
compounds and still thrive, producing tiny gold nuggets as a side-effect.
Just like many other elements, gold can move through what's known as a
biogeochemical cycle - being dissolved, shifted around, and eventually
re-concentrated in Earth's sediment.
Microbes are involved in every step of this process, which has led
scientists to wonder how they don't get poisoned by the highly toxic
compounds that gold ions usually form in the soil.
The rod-shaped C. metallidurans was first found to poop gold nuggets back in
2009, when scientists discovered that it somehow manages to ingest toxic
gold compounds and convert them into the element's metallic form without any
apparent danger to the organism itself.
"The results of this study point to their involvement in the active
detoxification of gold complexes leading to formation of gold biominerals,"
lead researcher, geomicrobiologist Frank Reith said in 2009.
Now, after years of investigation, Reith and his colleagues finally know the
precise mechanism of how the bacterium achieves this amazing feat.
C. metallidurans thrives in soils which contain both hydrogen and a range of
toxic heavy metals. This means the bacterium doesn't have much competition
from other organisms that can be easily poisoned in such an environment.
"If an organism chooses to survive here, it has to find a way to protect
itself from these toxic substances," says co-author of the latest study,
microbiologist Dietrich H. Nies from Martin Luther University
Halle-Wittenberg in Germany.
As it turns out, the bacterium has a pretty ingenious protective mechanism,
which involves not just gold, but also copper.
Compounds containing both of these elements can easily get into C.
metallidurans cells. Once inside, they interact in such a way that copper
ions and gold complexes get transported deep inside the bacterium, where
they could potentially wreak havoc.
To deal with this problem, bacteria employ enzymes to shift the offending
metals out of their cells - for copper, there's an enzyme called CupA. But
the presence of gold causes a new problem.
"When gold compounds are also present, the enzyme is suppressed and the
toxic copper and gold compounds remain inside the cell," says Nies.
At this point other bacteria might just give up and go live somewhere less
toxic, but not C. metallidurans. This organism has another enzyme up its
sleeve, which scientists have labelled CopA.
With this molecule, the bacterium can convert the copper and gold compounds
into forms that are less easily absorbed by the cell.
"This assures that fewer copper and gold compounds enter the cellular
interior," explains Nies.
"The bacterium is poisoned less and the enzyme that pumps out the copper can
dispose of the excess copper unimpeded."
But not only does this process let the microbe shed all that unwanted
copper, it also results in teeny tiny gold nugget nanoparticles on the
bacterial surface.
The results of this research, which builds on previous work by the same
team, are a fascinating insight into the workings of a strange microbe. But
on top of that, the bacterium's weird talent could actually be put to a good
use.
Understanding how C. metallidurans can poop out gold nuggets means
scientists just got a huge step closer to unlocking the biogeochemical cycle
of gold.
In the future these insights could be used to refine the precious metal from
ores that only contain small amounts of metal - something that's currently a
very tricky prospect.
The research has been published in Metallomics.
Reference:
L. Bütof, N. Wiesemann, M. Herzberg, M. Altzschner, A. Holleitner, F. Reith,
D. H. Nies. Synergistic gold–copper detoxification at the core of gold
biomineralisation in Cupriavidus metallidurans. Metallomics, 2018;
DOI: 10.1039/c7mt00312a