Scientists Develop Robust Thermophilic Cellulose Ethanol Bacterium

The production of cellulose ethanol from plant biomass usually goes through a series of steps involving (1) enzymatic breakdown of the complex cellulose/hemicellulose molecules of the plant biomass into their component sugars and (2) fermentation of the liberated sugars to ethanol (usually by yeasts). Although cellulose ethanol production has higher net energy yield and has a better carbon balance (compared to corn ethanol), its production cost is still high. Cellulose ethanol takes a longer time to produce compared to corn ethanol. The enzymes for cellulose/hemicellulose breakdown are still expensive. Furthermore, much of the non-glucose sugars in the fermentation broth (such as xylose from the breakdown of hemicelluloses) cannot be utilized by yeasts into ethanol. (Yeasts can only utilize the glucose in the broth to ethanol). Some of these hurdles are a step closer to being overcome, when scientists from Dartmouth College and Mascoma Corporation (United States) developed a metabolically engineered bacterium which can utilize the non-glucose sugars (xylose) in the fermentation broth into ethanol, at record breaking product yields and with little by-product formation at thermophilic (45 ^oC to 80 ^oC) temperatures. The metabolism of the bacterium, Thermoanaerobacterium saccharolyticum, was modified so that it can co-utilize glucose and xylose (and other sugars like mannose and arabinose) into ethanol at high product yield and with little by-products. The maximum ethanol titer produced by one strain was 37 g/liter, “the highest reported thus far for a thermophilic anaerobe”. Yeasts can reach only to as high as 10 to 12 g/L of ethanol. The research findings are reported in a recent issue of the Proceedings of the National Academy of Sciences (PNAS) (URL above).

Related information on thermophilic microorganisms and hemicelluloses: http://en.wikipedia.org/wiki/Thermophilic_bacteria
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=413821&blobtype=pdf
http://en.wikipedia.org/wiki/Hemi-cellulose