You may have heard the proclamation before: The next generation of biofuels will be derived from cellulosic plant material. And, in theory, this makes sense. Whereas ethanol can be produced via the fermentation of simple sugars in food crops such as corn or sugarcane, it would be more economical to make fuel from nonfood sources that are cheaper and more abundant—such as switchgrass, Miscanthus or wood chips. The problem? Today's methods for breaking down cellulose, the fibrous complex sugar that is the main structural component in green plants, are too expensive.
To degrade the tough plant material, engineers use enzymes isolated from organisms, such as termites, that rely on the molecular machines to convert their cellulose-rich meals into simpler, digestible sugars. But the enzymes currently available are not efficient enough to make cellulose-to-fuel conversion worthwhile. "If the industry is going to move forward, it's going to need new enzymes," says Eddy Rubin, the director of the U.S. Department of Energy's Joint Genome Institute. Rubin and 16 colleagues report in the January 28 issue of Science how they discovered nearly 30,000 new enzyme candidates by analyzing DNA collected from a cow's rumen—the first compartment in the animal's four-section stomach and home to a vast population of microbes equipped with potent enzymes that help digest the grasses their bovine host consumes.
The researchers employed a cow with a surgically placed tube, called a fistula, which allowed them direct access to the rumen. As Rubin explains, "Over millions of years, in exchange for housing in the cows, these organisms have gotten good at paying their rent by providing the host with broken down cellulose—sugars the cow can use as an energy substrate."
To gather rumen microbes so as to analyze their genetic material, the group placed nylon bags filled with switchgrass, the much-hyped next-generation biofuel feedstock, into the cow's rumen via the fistula. Plant-digesting organisms then "glommed on" to the switchgrass, and after 72 hours "we would pull the whole bag of material out and extract the DNA that was adherent to it," Rubin explains. This experimental setup was innovative compared with traditional in vitro methods for isolating enzymes in which microbes are grown in a laboratory incubator—a process that wouldn't have worked for these microbes, Rubin notes. "They're quite happy in the belly of the cow but they are not so happy living in your incubator."