Engineering sweetens biofuel prospects

A genetically engineered bacterium that produces high yields of ethanol from plant sugars could significantly lower the cost of biofuel production, according to a team of researchers from New Hampshire, US. The new strain works at high temperatures and does not produce any detectable by-products.

Second generation biofuels, made from agricultural waste such as straw, have been held back from mass production by the high cost of freeing plant sugars from their complex carbohydrate captors - lignin and cellulose.  Breaking down insoluble cellulose into fermentable sugars requires pre-treatment - for example, with steam or dilute acid - followed by addition of costly cellulase enzymes.

Thermophilic bacteria - which have evolved to thrive in hot environments such as geothermal hot springs - are a promising potential solution. They require less cellulase because of their high fermentation temperatures, and are very good at hydrolysing and fermenting biomass sugars to produce alcohol. They also display high growth rates and productivity. 

However, themophilic bacteria also produce organic acid by-products such as acetic acid, lowering the yield of ethanol. Lee Lynd and his team from Dartmouth College worked with US biofuels firm Mascoma Corporation to engineer Thermoanaerobacterium saccharolyticum, a thermophilic anaerobic bacterium. They knocked out the genes involved in organic acid production to produce a strain - named ALK2 - that produces ethanol as the only detectable organic product. 

’Strain ALK2 is better than conventional microbes at co-utilising different biomass sugars as well as short chains of these sugars, simultaneously fermenting  glucose and xylose [a C5 sugar from hemicelluloses],’ says Lynd.

Hotting up

  ALK2 also has a higher optimal temperature than conventional ethanol-producing microbes - 50 to 60?C compared with less than 37?C. ’The higher the fermentation temperature, the better,’ explains Lynd. ’Higher temperatures reduce the levels of enzyme required.’ In addition, both biomass pre-treatment and post-fermentation distillation is carried out at higher temperatures than fermentation - so higher fermentation temperatures mean less cooling prior to fermentation, and less heating after fermentation, adds Lynd.  

The concentrations of ethanol produced by strain ALK2 - about 4 per cent by weight - are higher than previously reported for thermophilic bacteria, but still only half the theoretical maximum yield. ’Closing the gap between the maximum concentrations of ethanol produced and maximum concentrations tolerated is still to be achieved for T. saccharolyticum,’ says Lynd.

Edward Green, chief technical officer at Green Biologics, a UK biotech firm that provides fermentation technology for biofuel production, is impressed by the US research: ’Thermophilic microbes are not easy to genetically manipulate, and the gene knockouts have had the desired effect without significantly impairing growth’. However, he is not convinced that Lynd’s team has chosen the best thermophile to work with. Facultative anaerobes, which flourish in the presence of oxygen but also survive without it, have more potential and higher growth rates, he says.

Lynd believes that the work paves the way to engineering a diverse group of bacteria to help improve ethanol yields. Meanwhile, he and his team are working on further improvements of ALK2 for industrial application. ’Strain ALK2 is under intensive development for commercial application by Mascoma,’ he adds.

Emma Davies

Enjoy this story? Spread the word using the ’tools’ menu on the left.