However, the genetically engineered bacteria cannot break down cellulose. In their laboratory experiments, Lynd and his coworkers needed to add enzymes to free the glucose from crystals of cellulose. Still, the bacteria offer an advantage because they are thermophilic–that is, they naturally grow at temperatures of 50 to 60 ºC. This is much higher than the 37 degrees at which yeast ferments sugars, and thus the bacteria require less fewer enzymes. “Because enzymes are more active at higher temperatures, using these bacteria would mean you have to add less enzyme,” Lynd says.
In the experiments, the bacteria fermented sugar mixtures at 50 ºC to give 4 percent ethanol concentration. “It’s the highest concentration of ethanol that’s been produced by thermophilic bacteria,” Lynd says.
Conventional yeast can give higher ethanol concentrations of 10 to 12 percent, says Harvey Blanch, a chemical-engineering professor at the University of California, Berkeley. Nevertheless, he says that the new work is a “nice proof of concept” for a combined approach to make cellulosic ethanol. While the researchers use cellulose crystals in their lab experiment, the challenge will be to see if the microbes can produce similar results with cellulosic biomass such as wood chips and switchgrass, says Blanch. “If this can be successfully accomplished, it will be a significant advance,” he says.
Lynd’s team is also trying to increase ethanol yield in a thermophilic bacteria that breaks down cellulose. The group wants to team it with the bacteria that are good at breaking down hemicellulose and using all sugars. That would give an all-in-one microbe system that breaks down biomass and converts all of its sugars into ethanol.
“Using one microbe or community of microbes for essentially the whole conversion process would be a major cost breakthrough,” says Anna Palmisano, associate director of science at the DOE’s Office of Biological and Environmental Research. “It’s one of the ways really fundamental biology could transform the equation and help pave the way to commercially viable cellulosic biofuels.”
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