Cellulose breakdown
The lignin barrier is not the only problem that needs to be solved before cellulosic ethanol can be made commercially, however. "You can think of lignin as a layer of paint," Daniel Cosgrove says. Once that's scraped off, there's the intractability of cellulose itself.
Daniel Cosgrove Credit James Collins
Cosgrove, who holds the Eberly chair in biology, uses a cardboard model to explain. He holds up a small white panel. "This is glucose, a nice sugar molecule, easily digested," he says. "Cellulose is basically glucose polymerized in a particular way." First, he demonstrates, the panels are joined end-to-end, one glucose linked to the next. Then—he pauses to fasten them—"these strips are cross-bonded together, into stiff sheets," Cosgrove says. "Next, these sheets of glucans stack up to make something like this." He holds out a stack several sheets deep. "That's cellulose."
"This is all just sugar," Cosgrove continues. "It ought to be useful for all kinds of organisms. But cellulose is really indigestible. It's fiber; that's what the nutritionists call it. Converting this substance into small sugars which can then be fermented into alcohol—that's the major technological hurdle."
Though he wasn't looking for it, Cosgrove happened on a possible solution over ten years ago when he discovered the family of plant proteins now known as expansins. Catalysts of cell growth, expansins have a unique loosening effect on plant cell walls.
"In trying to figure out how expansins work," Cosgrove says, "we discovered that if you add them to cellulose in the presence of cellulases—enzymes that break down cellulose—the cellulases become more efficient. It looks like expansins loosen up the structure of the cellulose." He picks up the stack of sheets, and removes the top sheet. "Our hypothesis is that expansins have the ability to lift off these glucans.
"We're continuing to try to understand how that happens—what the interaction is between expansins and cellulose—and also to explore various avenues for technological application, including biofuels," Cosgrove says. "One way would be to throw in expansins along with the cellulases and other enzymes that are currently used for digestion of cellulosic material.
"The other way would be to use genetic technology to get the plant to produce its own expansins, just before harvest. Maybe produce cellulases too, so you don't have to add them in," he says. "At this point we're testing the principle in arabidopsis and maize. If they look promising we'll put the gene into plants that are more likely candidates to be energy crops. But that's all a couple of years down the road."
On golden pond
One way to avoid the lignin problem altogether, Don Bryant suggests, would be to rely on microbial biomass as an energy feedstock. Cyanobacteria, often called blue-green algae, are hardy, fast-growing microbes found in oceans and freshwater ponds, indeed wherever there is moisture.
"You can directly ferment these organisms into ethanol," says Bryant, professor of biochemistry and molecular biology. "We could make big ponds out in Arizona or Nevada and grow tons of cyanobacteria if we can figure out a way to get water there."
But cyanobacteria and the other phototrophic organisms he studies are equally important, Bryant says, as models for the ultimate in renewable energy: photosynthesis. "These things are terrifically efficient—essentially every photon that goes in is converted into chemical energy," he says. "We'd like to be able to design man-made devices that had that kind of efficiency."
To that end, Bryant and his team study the light-harvesting designs of nature, particularly an antenna structure some phototrophs employ called a chlorosome. "It's literally a big sack of [light-absorbing] chlorophyll," he says. "Each cell of the principal organism we work with contains about 200 to 250 of these sacks, and each sack contains about 250,000 chlorophyll molecules.
"We want to understand everything we can about the design principles of these structures, how the molecules are synthesized," Bryant says. "On the surface it seems fairly simple—it's made up of chlorophyll aggregates—but actually it's made up of several different types of aggregates, in different proportions, under different growth conditions. There are modifications that change the size and shape and chirality of some of the side chains on these molecules.
"Turns out you these modifications provide some very important benefits for light-harvesting. We've worked out what several of those benefits are. But the extent to which that kind of information could be added into man-made devices is not really clear yet."
Grasses and trees
Back on dry land, Penn State researchers are investigating a wide variety of potential "second generation" feedstocks: crops developed and grown specifically for fuel. "Everybody talks about switchgrass as the new bioenergy crop," Tom Richard says. "Well, we've been doing research on switchgrass for almost 30 years, collaborating with USDA-ARS scientists based here at Penn State. But there are other grasses we should look at as well."
One candidate, giant miscanthus, is already being used as a biofuel in Europe. A perennial, "it grows extremely fast, and produces three times the biomass of switchgrass," Richard says. "How does it do it? What makes it more efficient? Can that mechanism be transfered to other plants?" In addition to their energy value, he notes, perennial grasses have environmental benefits, helping with carbon sequestration, soil erosion, wildlife habitats, and water quality. And unlike corn, they can be grown on marginal land.
John Carlson Credit James Collins
Another possibility is fast-growing trees. John Carlson is an expert on hybrid poplar, a tree well-studied for its value to the paper industry. As a member of the steering committee for the International Poplar Genome Consortium, Carlson helped spearhead the sequencing of the tree's genome in 2002.
Young poplar trees grow "easily three meters a year," Carlson says. "It is already grown in plantations." And its small genome makes it an ideal candidate for genetic engineering. Carlson has worked for years on modifying lignin biosynthesis in poplar and other trees, first for the paper industry and now for biomass energy.
Even without genetic engineering, he says, "we can use the knowledge of the poplar genome to improve traditional breeding for lower lignin and higher cellulose content." Other fast-growing trees like chestnut and yellow poplar might be even better biomass species in certain habitats, he adds.
"It's kind of a trade-off as to whether woody species are better than grasses for biomass applications," Carlson says. "Per rotation, trees produce much more biomass than grasses do, but grasses can be harvested annually." He has tried, unsuccessfully so far, to get funding to try alternating the two, switchgrass and hybrid poplar, on the same land.
"The idea would be to do this on mine reclamation sites in Pennsylvania," Carlson explains. "We could reclaim these damaged sites at the same time we're producing a crop. We could even use some of the excess cow manure from our dairy farms to supplement the soil, instead of having it end up in the Chesapeake Bay. These could be biomass plantations plantations with ecological restoration at the same time."