Scientists overcome major obstacles to cellulosic biofuel production

Dec 27, 2010
Illinois food science and human nutrition professor Yong-Su Jin (center), postdoctoral researcher Suk-Jin Ha (left), graduate student Soo Rin Kim and their colleagues engineered a yeast that outperforms the industry standard in the production of ethanol from cellulosic biomass. The effort involved researchers at Illinois, the Lawrence Berkeley National Laboratory, the University of California at Berkeley, Seoul National University and the oil company BP. Credit: L. Brian Stauffer, U. of I. News Bureau.

A newly engineered yeast strain can simultaneously consume two types of sugar from plants to produce ethanol, researchers report. The sugars are glucose, a six-carbon sugar that is relatively easy to ferment; and xylose, a five-carbon sugar that has been much more difficult to utilize in ethanol production. The new strain, made by combining, optimizing and adding to earlier advances, reduces or eliminates several major inefficiencies associated with current biofuel production methods.

The findings, from a collaborative led by researchers at the University of Illinois, the Lawrence Berkeley National Laboratory, the University of California and the energy company BP, are described in the . The Energy Biosciences Institute, a BP-funded initiative, supported the research.

Yeasts feed on sugar and produce various waste products, some of which are useful to humans. One type of yeast, Saccharomyces cerevisiae, has been used for centuries in baking and brewing because it efficiently ferments sugars and in the process produces ethanol and . The biofuel industry uses this yeast to convert plant sugars to bioethanol. And while S. cerevisiae is very good at utilizing , a building block of and the primary sugar in plants, it cannot use xylose, a secondary – but significant – component of the lignocellulose that makes up plant stems and leaves. Most yeast strains that are engineered to metabolize xylose do so very slowly.

"Xylose is a wood sugar, a five-carbon sugar that is very abundant in lignocellulosic biomass but not in our food," said Yong-Su Jin, a professor of food science and human nutrition at Illinois. He also is an affiliate of the U. of I. Institute for Genomic Biology and a principal investigator on the study. "Most yeast cannot xylose."

A big part of the problem with yeasts altered to take up xylose is that they will suck up all the glucose in a mixture before they will touch the xylose, Jin said. A glucose transporter on the surface of the yeast prefers to bind to glucose.

"It's like giving meat and broccoli to my kids," he said. "They usually eat the meat first and the broccoli later."

The yeast's extremely slow metabolism of xylose also adds significantly to the cost of biofuels production.

Jin and his colleagues wanted to induce the yeast to quickly and efficiently consume both types of sugar at once, a process called co-fermentation. The research effort involved researchers from Illinois, the Lawrence Berkeley National Laboratory, the University of California at Berkeley, Seoul National University and BP.

In a painstaking process of adjustments to the original yeast, Jin and his colleagues converted it to one that will consume both types of sugar faster and more efficiently than any strain currently in use in the biofuel industry. In fact, the new yeast strain simultaneously converts cellobiose (a precursor of glucose) and xylose to ethanol just as quickly as it can ferment either sugar alone.

"If you do the fermentation by using only cellobiose or xylose, it takes 48 hours," said postdoctoral researcher and lead author Suk-Jin Ha. "But if you do the co-fermentation with the cellobiose and xylose, double the amount of is consumed in the same amount of time and produces more than double the amount of ethanol. It's a huge synergistic effect of co-fermentation."

The new is at least 20 percent more efficient at converting xylose to ethanol than other strains, making it "the best xylose-fermenting strain" reported in any study, Jin said.

The team achieved these outcomes by making several critical changes to the organism. First, they gave the yeast a cellobiose transporter. Cellobiose, a part of plant cell walls, consists of two glucose sugars linked together. Cellobiose is traditionally converted to glucose outside the yeast cell before entering the cell through glucose transporters for conversion to ethanol. Having a cellobiose transporter means that the engineered yeast can bring cellobiose directly into the cell. Only after the cellobiose is inside the cell is it converted to glucose.

This approach, initially developed by co-corresponding author Jamie Cate at the Lawrence Berkeley National Laboratory and the University of California at Berkeley, eliminates the costly step of adding a cellobiose-degrading enzyme to the lignocellulose mixture before the yeast consumes it.

It has the added advantage of circumventing the yeast's own preference for glucose. Because the glucose can now "sneak" into the yeast in the form of cellobiose, the glucose transporters can focus on drawing xylose into the cell instead. Cate worked with Jonathan Galazka, of UC Berkeley, to clone the transporter and enzyme used in the new strain.

The team then tackled the problems associated with xylose metabolism. The researchers inserted three genes into S. cerevisiae from a xylose-consuming yeast, Picchia stipitis.

Graduate student Soo Rin Kim at the University of Illinois identified a bottleneck in this metabolic pathway, however. By adjusting the relative production of these enzymes, the researchers eliminated the bottleneck and boosted the speed and efficiency of xylose metabolism in the new strain.

They also engineered an artificial "isoenzyme" that balanced the proportion of two important cofactors so that the accumulation of xylitol, a byproduct in the xylose assimilitary pathway, could be minimized. Finally, the team used "evolutionary engineering" to optimize the new strain's ability to utilize xylose.

The cost benefits of this advance in co-fermentation are very significant, Jin said.

"We don't have to do two separate fermentations," he said. "We can do it all in one pot. And the yield is even higher than the industry standard. We are pretty sure that this research can be commercialized very soon."

Jin noted that the research was the result of a successful collaboration among principal investigators in the Energy Biosciences Institute and a BP scientist, Xiaomin Yang, who played a key role in developing the co-fermentation concept and coordinating the collaboration.

Explore further: Team advances genome editing technique

More information: The paper, "Engineered Saccharomyces cerevisiae capable of simultaneous cellobiose and xylose fermentation," is published in PNAS.

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jonnyboy
1 / 5 (1) Dec 27, 2010
WOW !!!!!!!
DamienS
5 / 5 (1) Dec 27, 2010
What's the endgame? Biologically derived fuel substitutes can never replace oil. All of these types of processes never scale well and to make any kind of a dent as a petroleum substitute, they would need to scale production by staggering amounts (which includes commensurate land use, plant cultivation and water use).
robbor
3.7 / 5 (3) Dec 27, 2010
damien s - the yeasts food would come in the form of agricultural straw and wood waste from the lumber industry both of which produce cellulose in the millions of tons. this form of fuel production will not be the holy grail but will be an important element of the alternative fuel landscape.
Dug
1 / 5 (1) Dec 27, 2010
The US gov. is so corrupted by big petro and ag dollars it just won't admit that 85% of human food comes from petro-chemically derived fertilizers and 95% of foods use petro fuels to get from field to consumer. Wastes can produce only 3% of the biofuels we need because the waste are either logistically or economically unfeasible. We all know that petroleum is a finite resource. We will run out - sooner than later. Do we really want to use the petroleum now to make bio-fuels (not to mention using the peak and essential food producing phosphate nutrients for bio-fuels that are estimated to be gone in 50 years) instead of using them for food production? Alternative energy development should focus on renewable sources like sunlight - and related forms of solar energy - wind, wave and tide. Using fertilizer to produce bio-fuels at scale does compete with human food - like we're doing with ethanol right now - and it is really, really stupid to the point of being suicida
blazingspark
5 / 5 (1) Dec 27, 2010
Dug.. No, phosphate nutrients are used in flowering plants, The Haber process provides nitrogen rich fertilizer. Crops like sugar-cane need more nitrogen than anything else. Also, Using plant derived fuels balances out the CO2 equation better than digging up more fossil fuels. Essentially this is solar power, and we need a convenient dense form of energy that is easy to store. It might not be optimal but it's a step in the right direction. Bacteria that efficiently go from sunlight+CO2-->ISO-Octane would be awesome. Ask Craig Venter about that...
DamienS
5 / 5 (1) Dec 28, 2010
damien s - the yeasts food would come in the form of agricultural straw and wood waste from the lumber industry both of which produce cellulose in the millions of tons. this form of fuel production will not be the holy grail but will be an important element of the alternative fuel landscape.

I understand, but what percentage replacement constitutes "an important element" and at what cost?

I can see this sort of thing being somewhat useful for small scale communities, where appropriate, but not as a significant contributor to global energy production (where millions of tons of feedstock would be a drop in the ocean).
Quantum_Conundrum
not rated yet Dec 28, 2010
damien s - the yeasts food would come in the form of agricultural straw and wood waste from the lumber industry both of which produce cellulose in the millions of tons. this form of fuel production will not be the holy grail but will be an important element of the alternative fuel landscape.


Do you have any idea how much wood chips, sawdust, and agricultural waste you'd need to even make a difference? Not to mention the fact most agricultural waste and food process waste is aleady used in animal feed for cattle, pigs, and chickens.

Based on the density of ethanol, if you cut down 1 in every thousand mature trees on the earth every year, and the entire mass of the tree was converted to ethanol(impossible,) that would be enough to replace our existing oil use for one year at current levels.

However, china, india, and several african and south american natons are increasing usage, even in spite of the "alternative energy" developments.
Quantum_Conundrum
not rated yet Dec 28, 2010
China will increase it's fossil fuel usage by about 50% in the next few decades, in spite of all their efforts into alternative energy. Some of the african or south american nations may increase their oil usage 5 or 10 times over during the next 50 years...

As for cutting down trees, we're already cutting down more every year than what grows back. If we were cutting down another 1/1000th of the mature trees every year, and increasing this number to keep up with growing fossil fuel demand, then we'd burn up everything on earth even faster.

If you replace the forests with sugar cane farms, which is only possible in some locations, this would be a joke. You'd need millions and millions of acres of cane farms world wide above what we already have,a nd I doubt there's even enough land area is suitable locations.
Quantum_Conundrum
not rated yet Dec 28, 2010
Yeah, it would take at least 10 or 20 million acres of sugar cane to produce enough biofuels each year to totally replace fossil fuels. Of course, this doesn't count the fuel costs of growing, harvesting, transporting, and processing the sugar cane. To be honest, I got this number using a completely absurdly optimistic assumption for average yield.

So we're talking about at LEAST 15,625 square miles of all-new sugar cane farms, if converting sugar to ethanol cost no energy at all.

Further, this doesn't account for disasters such as floods or droughts or pests, which means you need a much larger amount just as a safety net incase of such events.

So the most optimized, idealistic scenario I can come up with would need at a bare minimum 15,625 square miles of all-new sugar cane farms. And to put that in perspect, that's a land area over 1/4th the size of the state of Louisiana.

Where are you going to find that much suitable land that isn't already being used?
SteveL
not rated yet Dec 28, 2010
We pump, process or dig previously stored carbon-based energy from the Earth at an incredible rate. No single technology will be the silver bullet to our future energy needs. This is simply one small but important step for one process of the many that will be required to offset our present and future energy demands. There won't be any simple and easy solutions to our energy needs. It will take a lot of work and a lot of investment to develop multiple answers to this big issue. Different geological areas will require different solutions.

One of the best and easiest investments we can all do now is the reduction of energy consumption - but that gets beyond the scope of this article.
Quantum_Conundrum
not rated yet Dec 28, 2010
The only way this would be possible is if we grew a huge amount of our food hydroponically in high-rise buildings to save land area, and then used the land to grow biofuels on traditional farms.
mjesfahani
not rated yet Dec 30, 2010
yeah, oil suppliers countries have to leave!
SteveL
not rated yet Jan 04, 2011
The only way this would be possible is if we grew a huge amount of our food hydroponically in high-rise buildings to save land area, and then used the land to grow biofuels on traditional farms.


Where would hydroponics in a high rise get the solar energy needed for the plants? The plants would already need the amount of light expected to hit the building's footprint, making more than two levels of production require more energy than actually strikes the building - requiring supplemental energy for the plants, the water pumping, filtration and circulation systems not to mention maintaining PH and chemicals.

Unless you're talking mushrooms to flavor our Eloi... Expecting us to become Morlocks?