Abstract
Background: Clostridium thermocellum is a model thermophilic organism for the production of biofuels from lignocellulosic substrates. The majority of publications studying the physiology of this organism use substrate concentrations of ≤10 g/L. However, industrially relevant concentrations of substrate start at 100 g/L carbohydrate, which corresponds to approximately 150 g/L solids. To gain insight into the physiology of fermentation of high substrate concentrations, we studied the growth on, and utilization of high concentrations of crystalline cellulose varying from 50 to 100 g/L by C. thermocellum. Results: Using a defined medium, batch cultures of C. thermocellum achieved 93% conversion of cellulose (Avicel) initially present at 100 g/L. The maximum rate of substrate utilization increased with increasing substrate loading. During fermentation of 100 g/L cellulose, growth ceased when about half of the substrate had been solubilized. However, fermentation continued in an uncoupled mode until substrate utilization was almost complete. In addition to commonly reported fermentation products, amino acids - predominantly L-valine and L-alanine - were secreted at concentrations up to 7.5 g/L. Uncoupled metabolism was also accompanied by products not documented previously for C. thermocellum, including isobutanol, meso- and RR/SS-2,3-butanediol and trace amounts of 3-methyl-1-butanol, 2-methyl-1-butanol and 1-propanol. We hypothesize that C. thermocellum uses overflow metabolism to balance its metabolism around the pyruvate node in glycolysis. Conclusions: C. thermocellum is able to utilize industrially relevant concentrations of cellulose, up to 93 g/L. We report here one of the highest degrees of crystalline cellulose utilization observed thus far for a pure culture of C. thermocellum, the highest maximum substrate utilization rate and the highest amount of isobutanol produced by a wild-type organism.
Original language | English |
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Article number | 155 |
Journal | Biotechnology for Biofuels |
Volume | 7 |
Issue number | 1 |
DOIs | |
State | Published - 2014 |
Funding
PGT and LRL are affiliated with Mascoma Corporation, which partly funded this research. This work was supported by the BioEnergy Science Center (BESC). The BioEnergy Science Center is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science. Additional support was provided by Mascoma Corporation. The authors would like to acknowledge Professor Brian Jackson, Rodney Baker, and Emily Pierson at the Trace Elements Laboratory at Dartmouth College for the quantification of trace elements and Professor Willy Lambert at the Laboratory of Toxicology, Ghent University, for quantification of PABA. Notice: This manuscript has been authored by Dartmouth College under Subcontract Number 4000115284 and Contract Number DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript or allow others to do so, for United States Government purposes. This work was supported by the BioEnergy Science Center (BESC). The BioEnergy Science Center is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science. Additional support was provided by Mascoma Corporation. The authors would like to acknowledge Professor Brian Jackson, Rodney Baker, and Emily Pierson at the Trace Elements Laboratory at Dartmouth College for the quantification of trace elements and Professor Willy Lambert at the Laboratory of Toxicology, Ghent University, for quantification of PABA. Notice: This manuscript has been authored by Dartmouth College under Subcontract Number 4000115284 and Contract Number DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript or allow others to do so, for United States Government purposes.
Keywords
- 2,3-butanediol
- Amino acids
- Cellulose fermentation
- Clostridium thermocellum
- Fusel alcohols
- High solids
- Isobutanol