Engineering a microbial consortium for lignocellulosic biofuel production

Open Access
Author:
Zuroff, Trevor Roman
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 13, 2014
Committee Members:
  • Wayne Roger Curtis, Dissertation Advisor
  • Howard M Salis, Committee Member
  • Thomas Keith Wood, Committee Member
  • Katriona Shea, Committee Member
  • Manish Kumar, Committee Member
Keywords:
  • Lignocellulose
  • biofuels
  • Clostridium phytofermentans
  • cellulose
  • microbial consortia
Abstract:
Lignocellulose represents an abundant, renewable resource for the production of liquid transportation fuels to fulfill the energy demand of a growing, developing global population. However, the recalcitrant nature of the biomaterial has perplexed scientists for decades; lignocellulosic fuels still cost two to five times more to produce than their petroleum-derived counterparts. A novel class of bioprocess designs are necessary to overcome the costs associated with conversion of this obstinate feedstock. An intensified processing scheme, termed consolidated bioprocessing, was touted as an economically feasible bioprocessing strategy where consolidation of processing steps was thought to enhance both process and economic efficiencies. Yet no single organism has been discovered or engineered that is capable of efficient consolidated bioprocessing. On the other hand, nature executes what may be the ultimate consolidated bioprocess through conversion of plant material to carbon dioxide and methane as part of the global carbon cycle. The concerted action of numerous naturally-occurring microorganisms overcomes many of the issues encountered in industrial, single-organism lignocellulose bioprocessing. In this dissertation, based on a nature-inspired design, a two-member microbial consortium was developed for the conversion of cellulose to fuel. Cooperation is induced between the cellulolytic, anaerobic bacterium Clostridium phytofermentans and Saccharomyces cerevisiae via oxygen diffusion, controlling consortium populations and promoting fuel production. S. cerevisiae consumes oxygen to protect C. phytofermentans which in return degrades cellulose providing soluble carbon for the yeast. This symbiotic design improves ethanol and/or hydrocarbon production from purified cellulose over two-fold. Detailed analyses of C. phytofermentans biofilm formation and fermentation characteristics revealed that glucose and ethanol are robust inhibitors of C. phytofermentans surface attachment and growth, respectively. Ethanol tolerance was improved via strain evolution and metabolic engineering was applied to overcome an adaptive reduction in ethanol yield. Finally, a comprehensive mathematical framework was developed to model the consortium and provide insight into the intricate cooperation toward improving system productivity. In summary, inspired by naturally occurring microbial ecosystems, this dissertation describes the application of scientific and engineering principles to improve the viability of microbial consortia for industrial-scale conversion of lignocellulosic biomass to fuel.