DOE BESC engineered microbe improves biobutanol yield from cellulose by a factor of 10
14 August 2015
Researchers at the US Department of Energy’s (DOE’s) BioEnergy Science Center (BESC) have engineered a microbe that improves isobutanol yields from cellulose by a factor of 10. The work, published in the journal Metabolic Engineering, builds on results from 2011 in which researchers reported on the first genetically engineered microbe to produce isobutanol directly from cellulose. (Earlier post.)
Isobutanol is attractive because its energy density and octane values are closer to those of gasoline; it is useful not only as a direct replacement for gasoline but also as a chemical feedstock for a variety of products. For example, isobutanol can be chemically upgraded into a hydrocarbon equivalent for jet fuel.
While the earlier work by BESC researchers at DOE’s Oak Ridge National Laboratory and the University of California at Los Angeles was important from a proof-of-principle perspective, this new result represents a significant gain.
When we reported our initial finding four years ago, we were using Clostridium celluloyticium, which is a less complex organism from a metabolic engineering perspective. With this paper, we have successfully engineered similar traits in the much higher yielding Clostridium thermocellum, and that has taken us to new levels of consolidated bioprocessing efficiency.—co-author James Liao, UCLA Henry Samueli School of Engineering and Applied Science
Consolidated bioprocessing refers to the bundling of several processes in a single microbe that can be used to extract sugar from a plant’s cellulose and convert those sugars into a biofuel. This approach can be used to combine several steps—pretreatment, enzyme treatment and fermentation—to produce biofuel at a lower cost. The process also helps overcome the challenges of recalcitrance, or a plant’s natural defenses to being chemically dismantled. Recalcitrance is one of the primary economic barriers to using lignocellulosic biomass such as corn stover and switchgrass as a feedstock for biofuels.
While the previous genetically engineered microbe achieved conversion results of 0.6 gram of isobutanol per liter, Clostridium thermocellum has produced 5.4 g/L of isobutanol from cellulose in minimal medium at 50 ˚C within 75 h, corresponding to 41% of theoretical yield. Researchers accomplished this by inserting five genes into the microbe, enabling it to synthesize isobutanol.
Metabolic engineering for isobutanol production in C. thermocellum is hampered by enzyme toxicity during cloning, time-consuming pathway engineering procedures, and slow turnaround in production tests. In this work, we first cloned essential isobutanol pathway genes under different promoters to create various plasmid constructs in Escherichia coli. Then, these constructs were transformed and tested in C. thermocellum. Among these engineered strains, the best isobutanol producer was selected and the production conditions were optimized. We confirmed the expression of the overexpressed genes by their mRNA quantities. We also determined that both the native ketoisovalerate oxidoreductase (KOR) and the heterologous ketoisovalerate decarboxylase (KIVD) expressed were responsible for isobutanol production. We further found that the plasmid was integrated into the chromosome by single crossover. The resulting strain was stable without antibiotic selection pressure.—Lin et al.
The scientists view this as a clear next-generation advance over strategies that use yeast to create biofuels from cellulose.
In addition to this development, which moves the BESC team closer to the production goal of more than 20 grams per liter, the prospects of commercial realization of this approach are greatly enabled by the fact that the microbe works at temperatures high enough to keep competing bugs from contaminating the microbial fermentation tanks and interfering with the conversion process.—Paul Gilna, director of BESC
The authors noted that microbial engineering challenges remain, but they are encouraged by this finding. Other authors of the paper are Beth Papanek, Lauren Riley and Adam Guss of ORNL and Paul Lin, Lou Mi, Amy Morioka, Kouki Yoshino, Sawako Konishi and Sharon Xu of UCLA.
BESC, led by ORNL, is one of three DOE Bioenergy Research Centers established by the DOE’s Office of Science in 2007. The centers support multidisciplinary, multi-institutional research teams pursuing the fundamental scientific breakthroughs needed to make production of cellulosic biofuels, or biofuels from nonfood plant fiber, cost-effective on a national scale. The three centers are coordinated at ORNL, Lawrence Berkeley National Laboratory and the University of Wisconsin-Madison in partnership with Michigan State University.
Paul P. Lin, Luo Mi, Amy H. Morioka, Kouki M. Yoshino, Sawako Konishi, Sharon C. Xu, Beth A. Papanek, Lauren A. Riley, Adam M. Guss, James C. Liao (2015) “Consolidated bioprocessing of cellulose to isobutanol using Clostridium thermocellum,” Metabolic Engineering, Volume 31, Pages 44-52 doi: 10.1016/j.ymben.2015.07.001
Cellulosic Butanol would be an ideal biofuel: a better match for gasoline then ethanol and made from woody matter - what could go wrong?
Just hard to make, Anyway, well done and keep at it.
They can make 5.4 gms/L at 40% efficiency.
The goal is 20 gms/L, but this would yield an efficiency > 100% - or am I missing something.
Posted by: mahonj | 15 August 2015 at 01:25 AM
I was wondering the same thing!!! Somebody is doing some funny math because 20g/l would be 151.85% of the theoretical yeild LOL
Clearly the author missed something in the write up.
Posted by: DaveD | 15 August 2015 at 09:01 AM
Actually, Clostridium for butanol goes back to the work of Chaim Weizmann during WWI,which produced competing amounts of acetone from wood chips. Purity of the bacterial strain and temperature control were not major issues, which is remarkable. But for advances in petroleum synthesis, there would have been major strides before 1970 in biobutanol and other biofuels.
Dealing with reactions and product buildup in competition with butanol are issues, but you can bet that membrane and microtubule separation and remixing will keep up. If your typical bioethanol batch takes 30 days to make, the slow wait for biobutanol may not be so bad.
Posted by: kalendjay | 15 August 2015 at 01:23 PM
It's maybe something great but they will have a very hard time compete with conventional petroleum because of low available quantity of feedstock and the high cost of the reaction and process. I heard that there is some kind of tar sands that are not tapped in Utah u.s.a and everywhere else. These tar sands are dry contrary to humid tar sands in alberta Canada so cost few to process, I don't know why they are not tapped.
Posted by: gorr | 19 August 2015 at 08:56 AM
They'd go to 20 g/liter by increasing the cellulose loading. This increases the yield per unit fermenter volume, cutting costs.
So far I don't see anything about extracting isobutanol from the broth continuously. To do this would probably require extraction into another liquid; the boiling points are so close that distillation looks troublesome, and isobutanol has the higher boiling point so it would concentrate in the bottoms.
Posted by: Engineer-Poet | 19 August 2015 at 04:56 PM