Photo courtesy of Prof. Ludmilla Aristilde

Selective metabolic pathways for the utilization of different sugars in cellulose and hemicellulose.

January 31, 2017

Why Certain Bacteria Are Not So Efficient at Producing Biofuel

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NASA’s annual climate reports seem to be displaying a chilling trend: 2016 was the third consecutive hottest year on record. With the world’s fossil fuel consumption increasing by 0.6 percent last year, the chances of permanently altered climate patterns are no longer miniscule.

However, spurred by the Paris Agreement of 2015, countries seem to be embracing renewable sources of energy. Obstacles, such as their comparative efficiency, remain. That’s where a new study that sheds light on how bacteria metabolize biomass by Prof. Ludmilla Aristilde, biological and environmental engineering, could come in handy.

“For over a century, we’ve known that species in the genus Clostridium are known to have the metabolic potential to be efficient biofuel producers. When you want to have a biological platform to make fuel, you want to choose a platform that you know is amenable to doing so and improve upon it. That’s why I chose the bacterium Clostridium Acetobutylicum for this study,” Aristilde said. “The problem is that it’s very efficient at processing glucose into biofuel precursors but the challenge is to process different sugars simultaneously.

Typical plant waste consists of cellulose and hemicellulose which in turn consist of six and five carbon sugars, known as hexose and pentose, respectively. Aristilde observed that pentose sugars tended to be directed to the pathway used to manufacture this bacterium’s DNA and the carbon present was invested to create ribonucleotides, the precursors to DNA. Consequently, this makes it impossible for them to contribute to the production of chemicals typically used in biofuels. Meanwhile, certain hexose sugars are funnelled to glycolysis, a metabolic pathway that is linked to biofuel production.

“The interesting thing is that, when it was previously observed that there was little depletion of pentose sugars, it was hypothesized that the bacteria were not using them,” Aristilde said. “But, one reason you don’t see significant depletion of these sugars may be that you only need a certain amount to go into ribonucleotides, so the cell need only take out the required amount of these sugars.”

Aristilde used Liquid Chromatography-Mass Spectrometry, a technique used to measure the mass ratios of molecules of different charges, to follow the path of metabolites in the cell.

“Various sugars were labelled differently. Specifically, one sugar would be labelled with carbon isotopes, heavier than those naturally found in the sugar and the other sugar would be left unlabelled. Then using mass spectrometry, I followed the path of sugar carbons into different parts of the cell. As a result, I saw that glucose contributed to the creation of biofuel-associated metabolites but xylose did not,” Aristilde said.

The implications of the study are widespread. Because plant-based biofuels are carbon neutral, scientists are interested in ensuring that large-scale production of fuels is efficient. Aristilde argues that this study is a stepping stone to achieving such a reality because it enables better informed engineering decisions.

“Sometimes, we spend so much effort genetically engineering a microbe without completely understanding how it works and consequently attain only marginal improvements. This study should help us design bioreactors because we understand more about the metabolic capabilities and regulations in the cells we’re using.”