A study of the microbes that allow cows to digest grass could lead to better ways of making cellulosic biofuels.
Biofuels made from agricultural waste, sawdust, and prairie grass promise to be more economical than biofuels derived from corn, sugarcane, and other food crops.
The first step in cellulosic biofuels is converting tough plant materials made of cellulose and lignin into sugars that can then be fermented to make fuels. But this is expensive and currently requires a large quantity of enzymes to break down cellulose. “We’re talking truckloads,” says Frances Arnold, a professor of chemical engineering at Caltech who was not involved with the cow research. “We need a two- to fivefold reduction in the cost of enzymes,” she says.
In contrast, the microbes that live in the part of the bovine digestive tract called the rumen have been turning cellulose into sugar efficiently for millions of years. Researchers hope a new database of 28,000 genes sequenced from microbes involved in bovine digestion will help engineers come up with new enzymes, and bring down the cost of making cellulosic biofuels.
So far, manufacturers have brought down the costs of making cellulolytic enzymes mostly by changing processing methods. Another approach would be to make enzymes that work faster or work under different conditions, such as extreme temperatures, that might facilitate the breakdown of plant matter. “To begin to lower the costs of making cellulosic biofuels, we need new enzymes that do more,” says Eddy Rubin, director of the U.S. Department of Energy Joint Genome Institute. Rubin led the cow-microbe study.
The trouble is that an estimated 99.9 percent of all microbes on earth, including those in cow rumen, cannot be grown in culture in the lab. So bioprospectors looking for natural microbial enzymes with industrial promise have had a very limited pool of material to work with. Fortunately, new gene-sequencing technologies are changing that, allowing researchers to discover microbial enzymes by looking in their genes. Without having to grow microbes in the lab, researchers can sequence all the genetic material present in an entire ecosystem, then screen this data for genes of interest. This type of research is called metagenomics.
Rubin’s group started their search for better cellulolytic enzymes by studying termites in 2007. Microbes living in termite guts ferment woody roughage into sugars. The trouble with termites, Rubin says, was that “it was hard to get much material to work with” because termite guts are small. The studies didn’t generate many of the full-length genes needed to make working enzymes.