Engineered Microbes Boost Ethanol

Yeast that tolerates more ethanol than usual could bring down the cost of making the renewable fuel.

Researchers at MIT have created a new strain of yeast that tolerates high levels of ethanol and ferments sugars more efficiently, making more ethanol and doing it faster.

The advance could lead to smaller, cheaper ethanol manufacturing plants, as well as reduce the notoriously high amounts of energy needed to make ethanol.

The result, described in the current issue of Science, is also an important advance in the wider effort to make organisms that can convert cheap biomass, such as corn leaves and stalks, agricultural waste, and fast-growing plants including willow and switchgrass, into ethanol. Ethanol from such sources is widely agreed to be the key to making this biofuel economically competitive with fossil fuels.

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.