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A Sugar Cube, Please: I Need to Charge My Cellphone
September 18, 2003
By ANNE EISENBERG
LOTS of people shun sugar these days. It's out of favor in
popular low-carbohydrate diets.
But sugar may yet be redeemed. A small organism that feeds
on it can convert its calories not to flab on the midriff
but to a far more respectable product: a modest but steady
stream of electricity.
Two scientists at the University of Massachusetts have
discovered a novel sugar-loving micro-organism, Rhodoferax
ferrireducens, that may one day serve as a stable source of
low power.
"It's a sort of bacterial battery," said Derek R. Lovley,
an environmental microbiologist who led the research. The
results are reported in the current online issue of Nature
Biotechnology.
Dr. Lovley cultured the bug in an Amherst laboratory, far
from the aquifer in Oyster Bay, Va., where he found it.
Then he housed it in a simple two-compartment fuel cell. As
it fed on and metabolized sugar, the electrons freed in the
process accumulated on an electrode in the fuel cell,
producing a current. "It can transfer more than 80 percent
of the electrons available in the sugar," Dr. Lovley said,
"contrary to most previous microbial fuel cells that use
sugar and deliver in the range of 10 percent."
The bacterial battery might one day have many applications,
for example, in sensors in remote locations, or in
household devices that would draw on agricultural or other
sugar-based waste for power. Dr. Lovley's organism did its
job not only with sucrose, fructose and glucose - the
simple sugars found, for instance, in fruits, beets and
sugar cane - but also with xylose, a part of wood and
straw.
Many research groups in the United States and abroad are
working on biofuel cells that use microbes to convert
organic matter like sugar into electricity, said G. Tayhas
R. Palmore, an associate professor of engineering at Brown
University who does research on biofuel cells.
"People have been trying to make microbial fuel cells for
decades," she said, but have been vexed by low returns.
"Typically the bug uses all of the energy from the sugar to
grow and live," Dr. Palmore said, instead of giving up
electrons from the oxidative process. But Dr. Lovley's bug
is highly efficient. "He's found an organisms happy to give
its electrons to the electrode," she said.
Many microbial fuel cells increase their efficiency by
using a special compound to enter the organism, collecting
the electrons that accumulate and carting them off to an
electrode. Such mediators must typically be replenished.
"But Dr. Lovley's bug does the work all by itself," Dr.
Palmore said, "without the intermediate components we all
put in to facilitate electron transfer."
Dr. Lovley's fuel cell has an electrode at either end. As
it dines, the micro-organism converts the glucose solution
into carbon dioxide, simultaneously generating electrons
that are deposited on the electrode and travel through an
external circuit to the other electrode.
"We need only a small number of organisms because they gain
energy and rapidly increase in number," Dr. Lovley said. In
his laboratory the organism flourished, colonizing the
surface of the electrode and producing stable long-term
power for up to 25 days. The current, about 200 microamps
per square meter, was modest, about enough to run a
calculator.
Minor technical improvements increased its output. "When we
used graphite felt rather than rods for the electrodes, we
had an approximate threefold increase in current,'' he
said.
R. ferrireducens belongs to a group of micro-organisms Dr.
Lovley and colleagues have discovered only in the past few
years. Often described as iron-breathing, they use iron for
metabolic energy just as humans use oxygen to burn food.
"They live in an environment with no oxygen, but lots of
iron," Dr. Lovley said. "So they evolved the strategy of
iron respiration," grabbing carbon from sediment on the
seafloor and releasing carbon dioxide, then transferring
the electrons that accumulated to nearby iron oxides or
rust. "To the organism," he said, "the electrode in the
fuel cell probably looks like iron oxide, its usual
repository."
Leonard Tender, a scientist at the United States Naval
Research Laboratory in Washington, has worked in the past
with Dr. Lovley to identify an iron-breathing organism that
feeds on ocean sediment, releasing electrons from the mud.
"Our application was on the sea floor," he said. "Now he's
made it relevant to land-based applications," showing that
the new organism has the ability to generate electricity
efficiently from a widely available fuel. "Micro-organisms
sitting on the electrode can proliferate, so they could in
theory operate indefinitely as long as there's fuel," Dr.
Tender said.
Adam Heller, an emeritus professor of chemical engineering
at the University of Texas, is currently working on
miniature biofuel cells for low-power application in the
body. He said that Dr. Lovley's work continues a series of
experiments that began in the 1980's, when there was a hope
that cheap biomass could be used to generate electrical
power in fuel cells. "But these cells fell flat because the
cost was so high, and because of low power densities," he
said.
Microbial fuel cells like Dr. Lovley's might find
application in some specialty applications in military
research, he said. But microbial fuel cells intrinsically
have lower power densities than large-scale power
generation systems. "They have no application in systems
that generate power for the grid," he said.
Dr. Lovley agreed that his organism was not up to large
power applications but suggested that it might become one
of many localized power sources that people could use, like
solar panels on homes. "The power per person is not a lot,
but if you add it up," he said, "you might save some of the
requirements for power from other sources."
Dr. Palmore said that Dr. Lovley's discovery was a
breakthrough in coupling biological reactions with
electronics, highly suitable for applications like sensors.
"It's a big advancement," she said. "It's a phenomenal
thing to identify an organism that's evolved to function
with rust as an acceptor - it's an easy leap to go from a
rusty surface to an electrode."
http://www.nytimes.com/2003/09/18/technology/circuits/18next.html?ex=1064915540&ei=1&en=06689b0ece6bac23
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