Tackling the Riddle of Putting a Zebra in Your Tank William Widmer for The New York Times TRASH TO TREASURE David Mullin's focus is making butanol from the cellulose in waste like sugar cane mulch. NEW ORLEANS - Prowling the animal cages at the Audubon Zoo with tweezers and sandwich bags was an unusual and somewhat disconcerting experience for David A. Mullin and his graduate students. "I'd never stood next to a giraffe before," recalled Dr. Mullin, a professor of molecular biology at Tulane University. They're huge. Dr. Mullin was collecting manure from giraffes and other exotic ruminants at the zoo, a few blocks from his laboratory. He was looking for microbes capable of breaking down cellulose and converting it to biofuel. "Nature is very good at this," he said. So good that Dr. Mullin's lab machines these days are gently jiggling a dozen small glass containers filled with cellulose and TU-103, a bacterium he found in the droppings of an African zebra. To date, his microbe has thrived on cotton waste, a pure cellulose composed of fibers, twigs and seeds; on bagasse, the mulchlike residue of squeezed sugar cane; and on chopped-up copies of the city's main newspaper, The Times-Picayune. "Just cut it up, add water and put it in a Cuisinart," he said of the newsprint. It works fine. Dr. Mullin, who has no personal financial or commercial stake in the research, is one of many scientists and companies involved in the feverish competition to develop an economically viable biofuel substitute for gasoline. This is one of those frustrating scientific endeavors, like nuclear fusion, in which it is clear what needs to be done, but doing it in a sensible way has proved to be numbingly difficult. His research demonstrates both how simple the problem is and how complicated the solution might be. Making alcohol from sugars is easy; distillers have been doing it for centuries. But the trick with biofuels is to start not with sugar but with cellulose, the tough carbon latticework, indigestible to humans, that serves as plants" armor against predatory pests. After the cellulose is transformed into sugar, it can be fermented and distilled to make alcohol. The second trick is to do the whole job at a cost that makes it possible for a biofuel to compete with gasoline. And this is not easy. Processors for decades have used government subsidies to make ethanol from cornstarch and other sugars, but these are food products. Using them to make ethanol raises the demand for staple commodities like corn and cane sugar, keeping prices high. Refining cellulose, however, means using corn husks instead of corn kernels, bagasse instead of cane sugar, and agricultural and industrial waste of all kinds - things like wood chips, the leavings of paper mills and fallen leaves from the backyard. Breaking down cellulose is "the extra step, and it's proving to be a touchy and expensive step," said Harvey Blanch, a chemical engineer who is chief science officer for the Department of Energy's Joint BioEnergy Institute. We can do it today, but it's a question of cost. Economics is the driver. For that reason, some environmentalists are wary of biofuels. Making ethanol from sugars encourages farmers to put vast amounts of land into single-crop production, risking environmental damage from pesticides and fertilizers. And even if cellulose refining comes of age, skeptics worry that companies will simply buy up land to grow feedstock crops like switch grass or fast-growing poplar trees with the same harmful consequences. "We don't want to create new monocultures," said Sheila Karpf, legislative and policy analyst for the Environmental Working Group, a nonprofit advocacy group in Washington. We want to make sure it's sustainable and that there are enough environmental safeguards. The ideal biofuel, given these concerns, is one that can be made with anything scooped up off the ground or dumped directly into the fermenting vat from a trash bin. This is where researchers like Dr. Mullin come in. It was Hurricane Katrina that inadvertently led Dr. Mullin to seek diamonds in the dung. He was chairman of Tulane's biology department when the storm clobbered New Orleans in August 2005. The university lost power for three months; his experiments, all refrigerated, were destroyed. His department was so traumatized that a quarter of the staff resigned. "Up to then, I had always worked on theoretical projects, but Katrina made you see how things can fall apart," he said. Without power, not only can you not keep food, but you can't get money from an A.T.M., and you can't buy fuel even if you have a portable generator because the pump won't work. From then on, I decided to work only on practical problems.