Engineered Microbes Boost Ethanol

Researchers hope to engineer a single organism that will both break down the cellulose in these sources into sugars and ferment them to produce ethanol. The work by MIT chemical-engineering professor Gregory Stephanopoulos and his colleagues focuses on the second part of this process: fermenting sugars to make ethanol. The yeast strain they made can tolerate ethanol concentrations as high as 18 percent–almost double the concentration that regular yeast can handle without quickly dying. In addition, the new strain makes about 20 percent more ethanol by processing more of the glucose, and it speeds up fermentation by 70 percent.

Higher ethanol tolerance could lead to smaller, cheaper equipment for ethanol plants. Michael Ladisch, director of the laboratory of renewable-resources engineering at Purdue University, says ethanol fermentation is primarily carried out in tanks that have to be emptied and cleaned between batches. “The same volume of tank will make double the amount of ethanol in the same time period for a 10 percent final solution than for 5 percent, or the same amount of ethanol if the tank volume is halved,” he says. “If the fermentation proceeds 10 percent faster for the same final concentration, the reduction in tank volume would be 10 percent.” The higher concentrations also reduce the amount of water that must be removed in a final distillation step, thereby saving energy.

Improving yeast-ethanol tolerance is difficult because it is a complicated trait involving many genes. To tailor the expression of many genes at once, Stephanopoulos uses a process to induce random mutations in the genes for master regulator proteins. Each of these proteins controls the expression of multiple genes, so by altering them, Stephanopoulos sets off a cascade of changes in gene expression widespread enough to alter a trait like ethanol tolerance. The researchers randomly changed these proteins in a large yeast population, which led to some with an increased tolerance for ethanol.

So far the researchers have only modified a laboratory strain of the yeast, not the type now used in ethanol plants. “The next step is to show that this technology works with industry strains,” Stephanopoulos says.

If their approach can modify industrial yeast, it would drive down the cost of ethanol, Ladisch says. And eventually this research could have a wider impact, he says, because the mechanism that the researchers used to make the ethanol-tolerant yeast could be used as a blueprint to develop other wanted traits in microbes. “They now have a handle into fundamental metabolic pathways in how the yeast might be modified,” he says.

Stephanopoulos believes that cellulosic-ethanol yields could be improved by tailoring certain traits in microbes using his technique. It might be possible to make microbes that are tolerant of compounds other than ethanol that are created in the fermentation process and toxic to the microbes. He also hopes to produce strains that eat sugars with five carbon atoms, such as xylose, that are produced when cellulose is broken down. The microbes used in today’s processes only ferment sugars like glucose that have six carbon atoms.

Much work remains to be done to develop a single organism that can first break down cellulose into sugars before fermenting these sugars. Breaking down cellulose is a key constraint to the viability of making ethanol from biomass, says Lee Lynd, professor of engineering at Dartmouth College. “The research and development-driven advances with the greatest impact on producing cellulosic ethanol at low cost and high efficiency have to do with converting biomass into sugars,” he says.