NREL and BESC discovery explains higher biomass degrading activity of C. thermocellum; potential boon for cellulosic biofuels
06 February 2016
Researchers at the Energy Department’s National Renewable Energy Laboratory (NREL) and the BioEnergy Science Center (BESC) have discovered a new cell-free cellulosomal system in Clostridium thermocellum—the most efficient single biomass degrader characterized to date —that is not tethered to the bacterial cell wall and is independent of the primary (tethered) cellulosomes.
Their discovery was made during an investigation into the performance of C. thermocellum. The scientists found the microorganism utilizes the common cellulase degradation mechanisms known today (free enzymes and scaffolded enzymes—i.e., a structured architecture of enzymes—attached to the cell), and a new category of scaffolded enzymes not attached to the cell. Reported in an open-access paper in Science Advances, the finding could lead to cheaper production of cellulosic ethanol and other advanced biofuels.
The anaerobic bacterium C. thermocellum is a major candidate for the production of biofuels from biomass feedstocks because it already possesses both an external cellulase system and the internal metabolic pathways to convert biomass to ethanol. C. thermocellum is ubiquitous and has been isolated from soil, compost, herbivores, and hot springs.
C. thermocellum uses both a free-enzyme system and a tethered cellulosomal system (cellulosome) wherein carbohydrate active enzymes (CAZymes) are organized by primary and secondary scaffoldin proteins to generate large protein complexes attached to the bacterial cell wall.
These enzyme complexes are an amazing machinery. They can include up to 63 biomass-degrading enzymes. One can think of a cellulosome as a nanoscale octopus wrapping and digesting cellulose microfibrils from all angles.
—Yannick Bomble, project leader and senior author
BESC researchers at NREL used newly published cloning strategies, enabled by a collaboration with Dartmouth College, to probe the importance of the primary and secondary scaffoldins of C. thermocellum using scaffoldin deletion strains. They found the scaffoldins were essential to the cell wall defibrillation mechanism used by C. thermocellum. Native cellulosomes are capable of creating or at least maintaining increased substrate surface area during deconstruction by splaying and dividing the biomass particles. This ability is completely lost with any modification of these cellulosomes, such as the removal of the primary or secondary scaffoldins.
Using the same mutant strains as background, they also found a new type of enzyme assembly that is not tethered to the cell and allows the microorganism more freedom to explore for additional biomass or provides a redundancy in its cellulolytic system to assure a consistent source of sugars.
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| Model of C. thermocellum cellulase systems. (A to C) C. thermocellum cellulase systems consist of a free-enzyme system (A) and cellulosomal systems including both a cell-free cellulosomal system (B), which was discovered in this study, and cell-bound cellulosome systems (C), which were reported previously. Xu et al. Click to enlarge. |
The researchers suggested that the cell-free cellulosome complex can be seen as a “long range cellulosome” because it can diffuse away from the cell and degrade polysaccharide substrates remotely from the bacterial cell.
They found that the primary scaffoldin played the most important role in cellulose degradation by C. thermocellum, whereas the secondary scaffoldins have less important roles. Additionally, the distinct and efficient mode of action of the C. thermocellum exoproteome, wherein the cellulosomes splay or divide biomass particles, changes when either the primary or secondary scaffolds are removed, showing that the intact wild-type cellulosomal system is necessary for this essential mode of action.
Our mission is to enable and indeed accelerate the emergence of the cellulosic biofuels enterprise through our fundamental research. C. thermocellum is recognized as one of the most effective cellulose-degrading bacteria in the biosphere, thus the discovery of this new mode of action represents significant progress in the scientific underpinnings of advanced approaches for biofuel production.
—Paul Gilna, director of BESC
This discovery, enabled by the BioEnergy Science Center, will influence the strategies used to improve the cellulolytic activity of biomass degrading microbes going forward.
First author of the paper was NREL scientist Qi Xu. Others from NREL include Michael G. Resch, Kara Podkaminer, Shihui Yang, John O. Baker, Bryon S. Donohoe, Stephen R. Decker, Michael E. Himmel, and Yannick J. Bomble. Other authors from the BioEnergy Science Center are Charlotte Wilson, Dawn M. Klingeman, Daniel G. Olson, Richard J. Giannone, Robert L. Hettich, Steven D. Brown, Lee R. Lynd, and Edward A. Bayer.
Resources
Qi Xu, Michael G. Resch, Kara Podkaminer, Shihui Yang, John O. Baker, Bryon S. Donohoe, Charlotte Wilson, Dawn M. Klingeman, Daniel G. Olson, Stephen R. Decker, Richard J. Giannone, Robert L. Hettich, Steven D. Brown, Lee R. Lynd, Edward A. Bayer, Michael E. Himmel, Yannick J. Bomble (2016) “Dramatic performance of Clostridium thermocellum explained by its wide range of cellulase modalities” Science Advances doi: 10.1126/sciadv.1501254
The era of bio fuels for the world is long past and gone. In the past 2000 years most of the earths forests have disappeared and much of the soil destroyed. A German Nobel prize winning scientist has published the calculations that show that if the entire area of land now used for bio-fuels were converted to long life forests and the energy formerly produced on that land was replaced by fossil fuels there would be a net CO2 release reduction. US government figures indicate that 100 units of fossil fuel energy goes into the production of 123 units of ethanol energy; bio-ethanol, even from cellulose, is not anywhere near carbon neutral. Far more efficient use of fossil fuels has been demonstrated and practiced for years and eliminates any need for bio-fuels for automobiles and at far lower cost than any alternative. It still must be remembered that cellulostic ethanol is still food and can be converted to other less poisonous foods. We must stop now any practice that makes simple foods more expensive to any degree to the people of the earth so that we can pretend to be environmentally friendly with large vehicles. ..HG..
Posted by: Henry Gibson | 06 February 2016 at 12:07 PM
Henry,
The four cellulose ethanol plants in the mid west use corn stalks not trees.
They and others will soon make one BILLION gallons per year.
Ethanol is not a food, it is ethyl alcohol.
Posted by: SJC | 06 February 2016 at 02:04 PM
SJC,
Ethanol is one of my favorite foods! I like E40 aka 80 proof, aged in oak, from Bourbon County, Kentucky. Please drink responsibly.
Henry,
Don't blame deforestation on Biofuels. The American Plains never had forests only grass. Biofuel cultivation and production can greatly improve their efficiency reducing fossil fuel input to zero. If we drive BEV and PHEV we can reduce gasoline consumption by 90% and current Biofuel production could meet the remaining fuel requirement.
Posted by: Account Deleted | 07 February 2016 at 09:01 AM
Paper products do more to deforest the planet.
The irony is the native grass could be harvested to make biofuel.
Posted by: SJC | 07 February 2016 at 10:03 AM
Henry,
Your quote on fossil fuel input was incorrect. You stated "100 units of fossil fuel energy goes into the production of 123 units of ethanol energy".
The report actually states:
"0.78 million British thermal units (Btu) of fossil energy consumed for each 1 million Btu of ethanol delivered—compared to 1.23 million Btu of fossil energy consumed for each 1 million Btu of gasoline delivered" (https://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/ethanol_brochure_color.pdf).
You used the inverse so it would be 1/.78 units or 100 units of fossil energy to make 128 units of ethanol energy. However, ethanol uses 58% less input than gasoline (1.23 million btu/.78 million btu). The study also points out that cellulosic ethanol offers an 87% reduction in GHG emissions vs Corn based Ethanol.
Another misconception is that corn ethanol makes fuel instead of food. Ethanol is made from the fructose, the remaining product is distiller's grain a high protein animal feed. Animal feed is the principal use of Iowa corn. The fructose goes into soft drinks and other sweetened foods which is far worst for your health.
Posted by: Account Deleted | 07 February 2016 at 01:15 PM
Correction: On a per-gallon basis, GREET shows that corn ethanol could reduce GHG emissions by 18% to 28%; cellulosic ethanol offers an even greater benefit,
with an 87% reduction in GHG emissions.
Posted by: Account Deleted | 07 February 2016 at 01:19 PM
Brazil with sugarcane has a good yield and now they want to use the stalks for cellulose as well. We may not need liquid fuels much decades from now, but we need them between now and then. I would rather have them renewable than fossil and OPEC.
Posted by: SJC | 08 February 2016 at 08:33 AM