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Microbial duo boost biofuel production

Butanol fuel made from cultures of corn stalks, leaves and switchgrass

Researchers were able to produce 1.88 grams of isobutanol per litre of fluid - the highest concentration yet achieved in the conversion of plant material into biofuel (Source: Joseph Xu/Michigan Engineering Communications and Marketing)

Bio-boost Researchers have paired up a fungus and a bacteria to produce high-quality biofuel from agricultural waste materials such as corn stalks and leaves.

Using the fungus Trichoderma reesei and the common bacteria Escherichia coli, US researchers were able to produce 1.88 grams of the biofuel isobutanol, per litre of fluid, which represents the highest concentration yet achieved in the conversion of plant material into biofuel.

The results are published this week in the Proceedings of the National Academy of Sciences.

The researchers chose isobutanol as their desired biofuel, rather than the more common ethanol, because isobutanol gives off 82 per cent of the heat energy petrol provides when burned, compared to ethanol's 67 per cent. Unlike ethanol, isobutanol doesn't easily absorb water, which can corrode pipelines and damage engines.

They also took the relatively new approach of using two types of microbes rather than one.

"Previously people have tried to engineer one microbial species or strain to do all of these things including biomass degradation and the fermentation for making the fuel molecules, and this has turned out to be really difficult," says Professor Xiaoxia Nina Lin, assistant professor at the University of Michigan's chemical engineering department.

"It's actually not surprising because what you want to do here are very different things so they happen in different organisms naturally but if you want to have them appear in the same organism, it is very hard to engineer."

In this study, the fungus T. reesei breaks down the tough feedstock into sugars, and the E. coli then converts those into the desired fuel.

However coaxing the two microbes to cooperate is no easy task as co-cultures are notoriously fragile and unstable.

Co-operators and cheaters

The key to this success was finding microbes that in some way depended on each other - or in this case, one microbe acts as the co-operator while the other is more of a 'cheater' says Lin.

"In our system, we have one microbe depending on the other for making the food sugars, but then the other one, what we call the cheater one, actually doesn't do anything for the co-operator," she says.

"We rely on the second one to make the fuels, but in the system itself, it's actually acting like a cheater by taking away the food which was made by the first guy."

However the co-operator - the fungus - still gets something out of the process because as part of converting the cellulose to sugars, it concentrates the food around itself.

Another advantage of the process is that it can use a range of waste products including woodchips from forestry or even fast-growing grasses.

The researchers are now focused on improving the efficiency of the process, which currently delivers 62 per cent of the theoretical maximum yield.

"It's still relatively low compared to ethanol - with ethanol, people can go over 90 per cent of the theoretical yield - so this is still one of the things that we are trying now, to make the process better," Lin says.

"It was a good initial demonstration of the concept but we're really interested in bringing this technology to the real world, so we have a lot to do."

Tags: alternative-energy, chemistry, energy