Newswise — Researchers
at the University of Massachusetts Amherst have discovered
a tiny biological structure that is highly electrically
conductive. This breakthrough helps describe how
microorganisms can clean up groundwater and produce
electricity from renewable resources. It may also
have applications in the emerging field of nanotechnology,
which develops advanced materials and devices in
extremely small dimensions.
The
findings of microbiologist Derek R. Lovley's research
team are published in the June 23rd issue of Nature,
an international science journal. Researchers found
that the conductive structures, known as “microbial
nanowires,” are produced by a novel microorganism
known as Geobacter. The nanowires are incredibly
fine, only 3-5 nanometers in width (20,000 times
finer than a human hair), but quite durable and more
than a thousand times long as they are wide.
“Such long, thin conductive structures are unprecedented
in biology,” said Lovley. “This completely changes
our concept of how microorganisms can handle electrons,
and it also seems likely that microbial nanowires
could be useful materials for the development of
extremely small electronic devices.”
“The microbial world never stops surprising us,” said
Dr. Aristides Patrinos of the U.S. Department of
Energy, which funds the Geobacter research. “The
remarkable and unexpected discovery of microbial
structures comprising microbial nanowires that may
enable a microbial community in a contaminated waste
site to form mini-power grids could provide new approaches
to using microbes to assist in the remediation of
DOE waste sites; to support the operation of mini-environmental
sensors, and to nano-manufacture in novel biological
ways. This discovery also illustrates the continuing
relevance of the physical sciences to today's biological
investigations.”
Eugene
Madsen, a Cornell University research microbiologist,
noted, “I have watched and judged, in peer review,
many of Dr. Lovley's remarkable scientific advancements
since the discovery of Geobacter in 1987. The latest
advancement, microbial nanowires, is another major
milestone because it may usher in a new era of exploration
of both microbial respiration and bio-electronics.” The
findings, he said, are “promising and exciting,” although
he emphasized the information must be independently
confirmed and extended by other microbiologists and
biophysicists.
Geobacter are the subject of intense investigation
because they are useful agents in the bioremediation
of groundwater contaminated with pollutants such
as toxic and radioactive metals or petroleum. They
also have the ability to convert human and animal
wastes or renewable biomass into electricity. To
carry out these processes, Geobacter must transfer
electrons outside the cell onto metals or electrodes.
This new research provides an explanation of how
this can happen.
Previous studies in Lovley's laboratory demonstrated
that Geobacter produces fine, hairlike structures,
known as pili, on just one side of the cell. Lovley's
team speculated that the pili might be miniature
wires extending from the cell that would permit Geobacter
to carry out its unique ability to transfer electrons
outside the cell onto metals and electrodes. This
was confirmed in a study in which microbiologist
Gemma Ruegera teamed with physicists Mark T. Tuominen
and Kevin D. McCarthy to probe the pili with an atomic
force microscope. They found the pili were highly
conductive. Furthermore, when Geobacter was genetically
modified to prevent it from producing pili, Geobacter
could no longer transfer electrons.
“These results help us understand how Geobacter
can live in environments that lack oxygen and carry
out such unique phenomena as removing organic and
metal pollution from groundwater,” Lovley said. Geobacter
can live in the absence of oxygen because of its
ability to transfer electrons outside the cell onto
iron minerals, which are natural constituents of
most soils. However, prior to the discovery of its
conductive pili it was unknown how this electron
transfer might take place.
The conductive pili that Geobacter produces may
have a variety of applications for the electronics
industry. Ultrafine wires, often referred to as nanowires,
are required for further miniaturization of electronic
devices. Manufacturing nanowires from more traditional
materials such as metals, silica, or carbon is difficult
and expensive. However, it is easy to grow billions
of Geobacter cells in the laboratory and harvest
the microbial nanowires that they produce. Furthermore,
by altering the DNA sequence of the genes that encode
for microbial nanowires, it may be possible to produce
nanowires with different properties and functions.
Another interesting implication of this research
is that it suggests a mechanism for microbes to share
energy in a mini-power grid. The nanowire pili of
individual Geobacter often intertwine, suggesting
a strategy by which Geobacter might share electricity.
Geobacter was discovered by Lovley in 1987 at the
muddy bottom of the Potomac River in Washington D.C.,
and over the past 18 years his research has earned
widespread media attention and major funding from
government and private sources. The tiny organisms,
widely found in soils and aquatic sediments, have
demonstrated promise as cleaners of toxic spills
and generators of energy. They are anaerobic bacteria
(living without oxygen) that use metals to gain energy
the way humans and other organisms use oxygen. They
are distributed throughout the world in a wide variety
of soils and sediments. Geobacter have been used
to help remove contaminants from underground petroleum
spills and landfill pollution of groundwater, as
well as remove uranium from contaminated groundwater
at a number of U.S. Department of Energy sites.
The title of the paper published in Nature is “Extracellular Electron Transfer
Via Microbial Nanowires.” The authors are Derek R. Lovley, Gemma Reguera, Teena
Mehta and Julie S. Nicoll of the UMass Amherst Department of Microbiology;
and Kevin D. McCarthy and Mark T. Tuominen of the UMass Amherst Department
of Physics.
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