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 ethanol. 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 Cambridge, 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 difficult. For decades, scientists have known of bacteria that can degrade cellulose and also produce some ethanol. Yet none can do the job quickly and efficiently enough to be useful for large-scale manufacturing.
Nature, Arnold explains, offers 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 different 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 off the bottom of their shoe.”
She didn’t quite scrape it off 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 several 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 components of the plant, and it forms ethanol as the main product,” she says. “It produces prodigious amounts of ethanol.”
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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 different, 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 discovered: 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.”