A team led by Dr. Janet Westpheling at the University of Georgia has engineered the thermophilic, anaerobic, cellulolytic bacterium Caldicellulosiruptor bescii, which in the wild efficiently uses un-pretreated biomass—to produce ethanol from biomass without pre-treatment of the feedstock. A paper on the work is published in Proceedings of the National Academy of Sciences (PNAS).
In January, Dr. Westpheling and her colleagues reported in the journal Science their discovery that an enzyme (the cellulase CelA) from C. besciia can digest cellulose almost twice as fast as Cel7A, the current leading component cellulase enzyme on the market. (Earlier post.)
The UGA research group engineered a synthetic pathway into the organism, introducing genes from other anaerobic bacterium that produce ethanol, and constructed a pathway in the organism to produce ethanol directly. Westpheling spent two and a half years developing genetic methods for manipulating the C. bescii bacterium to make the current work possible.
Here we report the direct conversion of switchgrass, a nonfood, renewable feedstock, to ethanol without conventional pretreatment of the biomass. This process was accomplished by deletion of lactate dehydrogenase and heterologous expression of a Clostridium thermocellum bifunctional acetaldehyde/alcohol dehydrogenase. Whereas wild-type C. bescii lacks the ability to make ethanol, 70% of the fermentation products in the engineered strain were ethanol [12.8 mM ethanol directly from 2% (wt/vol) switchgrass, a real-world substrate] with decreased production of acetate by 38% compared with wild-type. Direct conversion of biomass to ethanol represents a new paradigm for consolidated bioprocessing, offering the potential for carbon neutral, cost-effective, sustainable fuel production.—Chung et al.
Pre-treatment of biomass feedstock has long been the economic bottleneck hindering fuel production from lignocellulosic biomass feedstocks.
Given a choice between teaching an organism how to deconstruct biomass or teaching it how to make ethanol, the more difficult part is deconstructing biomass. Now, without any pretreatment, we can simply take switchgrass, grind it up, add a low-cost, minimal salts medium and get ethanol out the other end. This is the first step toward an industrial process that is economically feasible.—Janet Westpheling
Caldicellulosiruptor bacteria have been isolated around the world—from a hot spring in Russia to Yellowstone National Park. Westpheling explained that many microbes in nature demonstrate prized capabilities in chemistry and biology but that developing the genetic systems to use them is the most significant challenge.
Ethanol is but one of the products the bacterium can be taught to produce. Others include butanol and isobutanol, as well as other fuels and chemicals—using biomass as an alternative to petroleum.
Westpheling’s co-authors on the paper are Daehwan Chung and Minseok Cha of the Franklin College department of genetics and Adam M. Guss of the Oak Ridge National Laboratory. All authors are members of the DOE BioEnergy Science Center headquartered at Oak Ridge National Laboratory, Oak Ridge, Tennessee. BESC is a US Department of Energy Bioenergy Research Center supported by the DOE Office of Science.
Daehwan Chung, Minseok Cha, Adam M. Guss, and Janet Westpheling (2014) “Direct conversion of plant biomass to ethanol by engineered Caldicellulosiruptor bescii,” PNAS doi: 10.1073/pnas.1402210111