Increasing the production of biofuels like ethanol could be an important step in reducing the global consumption of fossil fuels. However, ethanol production is limited in large part by reliance on corn, which is not grown in sufficient quantities to meet a significant portion of US fuel needs.
To increase the potential impact of biofuels, a team of MIT engineers have now found a way to expand the use of a wider range of non-food raw materials to make such fuels. At the moment, raw materials such as straw and trees are difficult to use for biofuel production because they first have to be broken down into fermentable sugars.
MIT researchers devised a way to circumvent this toxicity by making it possible to use these much more abundant sources to make biofuels. They also showed that this tolerance can be built into yeast strains used to make other chemicals, potentially allowing “cellulosic” woody plant material to be used as a source for making biodiesel or bioplastics.
“We want to open up cellulose raw materials for almost every product and use the sheer abundance of cellulose,” says Felix Lam, research fellow at MIT and first author of the new study.
Gregory Stephanopoulos, Willard Henry Dow Professor in Chemical Engineering, and Gerald Fink, Margaret and Herman Sokol Professor at the Whitehead Institute of Biomedical Research and American Cancer Society Professor of Genetics in the Department of Biology at MIT, are the lead authors of the article, published today in Science Advances appears.
Increase in tolerance
Around 40 percent of the US corn crop is currently in ethanol. Maize is primarily a food crop that requires plenty of water and fertilizer, so plant material known as cellulose biomass is considered an attractive, non-competing source of renewable fuels and chemicals. This biomass, which includes many types of straw and parts of the corn plant that normally go unused, could make up more than 1 billion tons of material per year, according to a study by the US Department of Energy – enough to use 30 to 50 percent of the oil for transportation .
However, two main obstacles to using cellulosic biomass are that cellulose must first be released from the woody lignin and then the cellulose must be broken down further into simple sugars that yeast can use. The particularly aggressive preprocessing creates compounds, so-called aldehydes, which are very reactive and can kill yeast cells.
To overcome this, the MIT team built on a technique it had developed a few years ago to improve the tolerance of yeast cells to a wide variety of alcohols that are also toxic to yeast in large quantities. In this study, they showed that equipping the bioreactor with specific compounds that strengthen the yeast membrane helps the yeast to survive much longer in high concentrations of ethanol. Using this approach, they were able to improve the conventional fuel-ethanol yield of a high-performing yeast strain by about 80 percent.
In their new study, the researchers engineered yeast to convert the cellulosic by-product aldehydes into alcohols, thereby leveraging their already developed alcohol tolerance strategy. They tested several naturally occurring enzymes from different types of yeast that perform this reaction and identified one that worked best. Then they used directed evolution to improve it further.
“This enzyme converts aldehydes into alcohols, and we have shown that the other methods we have developed can make yeast much more tolerant of alcohols as a class than it is of aldehydes,” says Stephanopoulos.
Yeasts are generally not very efficient at producing ethanol from toxic cellulosic feedstocks; However, when the researchers expressed this top performing enzyme and added the membrane-reinforcing additives to the reactor, the strain tripled its cellulosic ethanol production to levels consistent with conventional corn ethanol.
Rich raw materials
The researchers showed that using five different types of cellulosic feedstock, including switchgrass, wheat straw, and corn stalks (the leaves, stems and husks that remain after the corn is harvested), they can achieve high ethanol yields.
“With our engineered strain, you can get maximum cellulose fermentation from all of these normally very toxic raw materials,” says Lam. “The great thing is that it doesn’t matter if your leftover corn might not be that great in a season. You can switch to energy straws or if you don’t have high availability of straws you can switch to some sort of pulpy, woody residue. “
The researchers also built their aldehyde-to-ethanol enzyme into a strain of yeast that was modified to produce lactic acid, a precursor to bioplastics. As with ethanol, this strain was able to produce the same yield of lactic acid from cellulosic materials as it did from corn.
This demonstration suggests that it might be possible to build aldehyde tolerance into yeast strains that produce other products such as diesel. Biodiesel could potentially have a huge impact on industries such as heavy haulage, shipping, and aviation, which lack an emission-free alternative like electrification and which require huge amounts of fossil fuels.
“Now we have a tolerance module that can be screwed onto almost any production path,” says Stephanopoulos. “Our goal is to extend this technology to other organisms that are better suited to producing these heavy fuels such as oils, diesel and kerosene.”
The research was funded by the US Department of Energy and the National Institutes of Health.
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