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stock, cellulosic biomass could greatly increase the energy and
environmental benefits of biofuels. It takes far less energy to grow
cellulosic materials than to grow corn, and portions of the biomass
can be used to help power the production process. (The sugarcane-
based ethanol produced in Brazil also o ers improvements over
corn-based ethanol, thanks to the crop's large yields and high
But despite years of research and recent investment in scaling up
production processes, no commercial facility yet makes cellulosic
ethanol. The economic explanation is simple: it costs far too much
to build such a facility. Cellulose, a long-chain polysaccharide that
makes up much of the mass of woody plants and crop residues such
as cornstalks, is di cult---and thus expensive---to break down.
Several technologies for producing cellulosic ethanol do exist.
The cellulose can be heated at high pressure in the presence of
oxygen to form synthesis gas, a mixture of carbon monoxide
and hydrogen that is readily turned into ethanol and other fuels.
Alternatively, industrial enzymes can break the cellulose down
into sugars. The sugars then feed fermentation reactors in which
microörganisms produce ethanol. But all these processes are still
far too expensive to use commercially.
Even advocates of cellulosic ethanol put the capital costs of
constructing a manufacturing plant at more than twice those for
a corn-based facility, and other estimates range from three times
the cost to five. "You can make cellulosic ethanol today, but at a
price that is far from perfect," says Christopher Somerville, a plant
biologist at the University of California, Berkeley, who studies how
cellulose is formed and used in the cell walls of plants.
"Cellulose has physical and chemical properties that make it
di cult to access and di cult to break down," explains Caltech's
Arnold, who has worked on and o on the biological approach to
producing cellulosic ethanol since the 1970s. For one thing, cel-
lulose fibers are held together by a substance called lignin, which
is "a bit like asphalt," Arnold says. Once the lignin is removed, the
cellulose can be broken down by enzymes, but they are expensive,
and existing enzymes are not ideal for the task.
Many researchers believe that the most promising way to make
cellulosic biofuels economically competitive involves the creation---
or the discovery---of "superbugs," microörganisms that can break
down cellulose to sugars and then ferment those sugars into etha-
nol. The idea is to take what is now a multistep process requiring
the addition of costly enzymes and turn it into a simple, one-step
process, referred to in the industry as consolidated bioprocessing.
According to Lee Lynd, a professor of engineering at Dartmouth
College and cofounder of Mascoma, a company based in Cam-
bridge, MA, that is commercializing a version of the technology,
the consolidated approach could eventually produce ethanol at
70 cents a gallon. "It would be a transformational breakthrough,"
he says. "There's no doubt it would be attractive."
But finding superbugs has proved di cult. For decades, scien-
tists have known of bacteria that can degrade cellulose and also
produce some ethanol. Yet none can do the job quickly and e -
ciently enough to be useful for large-scale manufacturing.
Nature, Arnold explains, o ers little help. "There are some
organisms that break down cellulose," she says, "but the problem
is that they don't make fuels, so that doesn't do you much good."
An alternative, she says, is to genetically modify E. coli and yeast so
that they secrete enzymes that degrade cellulose. But while many
di erent kinds of enzymes could do the job, "most them don't like
to be inserted into E. coli and yeast."
Arnold, however, is optimistic that the right organism will be
discovered. "You never know what will happen tomorrow," she
says, "whether it's done using synthetic biology or someone just
scrapes one o the bottom of their shoe."
She didn't quite scrape it o her shoe, but Susan Leschine,
a microbiologist at the University of Massachusetts, Amherst,
believes she just might have stumbled on a bug that will do the
job. She found it in a soil sample collected more than a decade
ago from the woods surrounding the Quabbin Reservoir, about
15 miles from her lab. The Quabbin sample was just one of many
from around the world that Leschine was studying, so it was sev-
eral years before she finished analyzing it. But when she did, she
realized that one of its bacteria, Clostridium phytofermentans, had
extraordinary properties. "It decomposes nearly all the compo-
nents of the plant, and it forms ethanol as the main product," she
says. "It produces prodigious amounts of ethanol."
Leschine founded a company in Amherst, SunEthanol, that will
attempt to scale up ethanol production using the bacterium. There's
"a long way to go," she acknowledges, but she adds that "what we
have is very di erent, and that gives us a leg up. We already have a
microbe and have demonstrated it on real feedstocks." Leschine
says that other useful microbes are probably waiting to be discov-
ered: a single soil sample, after all, contains hundred of thousands
of varieties. "In this zoo of microbes," she says, "we can think that
there are others with similar properties out there."
Whether ethanol made from cellulosic biomass is good or bad for
the environment, however, depends on what kind of biomass it
is and how it is grown.
In his o ce in St. Paul, David Tilman, a professor of ecology
at the University of Minnesota, pulls out a large aerial photo of a
field sectioned into a neat grid. Even from the camera's vantage
point far above the ground, the land looks poor. In one plot are
thin rows of grasses, the sandy soil visible beneath. Tilman says
the land was so infertile that agricultural use of it had been aban-
doned. Then he and his colleagues scraped o any remaining top-
soil. "No farmer has land this bad," he says.
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