| dc.description.abstract | In view of the realization that fossil fuels reserves are limited, the dramatically increasing consumption of fossil fuels raised concerns for the energy security in the recent decades. Alternative energy generating options are being explored. Biological methods for producing sustainable biofuels converted from cellulose such as hydrogen and bio-ethanol have the potential to provide a sustainable method to meet the requirements of the new generation energy. However, huge challenges still exist to improve the efficiency of cellulose bioconversion process and thus reduce the economic cost to enable large-scale application of biofuels using this strategy. This study aimed to improve the bioconversion process of cellulose to produce biofuels including hydrogen and ethanol, by investigating the biofuel production of mesophilic and thermophilic Clostridia under different conditions and revealing the mechanisms behind the high bioconversion efficiency.
Hydrogen gas production via dark fermentation of cellulose has been investigated as a potential source of renewable energy. The model strain of mesophilic Clostridium species, Clostridium cellulolyticum, is capable of both cellulose hydrolysis and H2 production, which make it a potential candidate in producing hydrogen from cellulose under a Consolidated Bioprocessing (CBP) scheme. However, it has been reported that C. cellulolyticum was considered as a sluggish cellulolytic bacterium, which means that the efficiency of bioconversion of cellulose still need to be improved. In the beginning of this study, the effects of different initial cellulose concentrations on cellulose degradation rates and hydrogen productions of C. cellulolyticum were examined. The results indicated that culturing C. Cellulolyticum on slow released substrates (e.g., cellulose but not cellobiose) can reduce the accumulation of intermediate products (e.g., glucose and/or pyruvate). The restriction of substrate availability can balance the metabolism rate of intermediate products in C. Cellulolyticum, which can relieve it from catabolite repression and improve the hydrogen production efficiency. Further transcriptional analysis indicated that cellulosomal genes were down-regulated along with the increase of cellulose concentrations, however, the expression level of other genes related to central metabolic pathway peaked at 7 g/L cellulose. Our study agreed well with previous studies and provided detailed transcriptional information of the carbon metabolism of C. cellulolyticum.
To further improve the hydrogen production and cellulose degradation, a co-cultured consortium composed of C. cellulolyticum and Desulfovibrio vulgaris Hildenborough was developed and it was optimized to achieve the best possible hydrogen production rate in this study. The co-culture can produce 3.3 mol H2 mol-1glucose comparing to the 1.8 mol H2 mol-1 glucose from the mono-culture with a much more efficient cellulose degradation process. Our results suggested that lactate may be the carbon source that C. cellulolyticum provides to D. vulgaris Hildenborough by degrading cellulose, so the lactate concentration in the co-culture can be kept at a low level and the catabolic pathway of lactate production in C. cellulolyticum will keep going and result in low NADH/NAD+ ratios. This efficient regulation of carbon flow of C. cellulolyticum enable the strain to achieve a high cell abundance in the co-culture, which in turn, promotes the efficient cellulose degradation in the co-culture system. Further analysis indicated that D. vulgaris Hildenborough does not only use lactate to produce H2 in the system, but also be able to help C. cellulolyticum attach on cellulose fibers to speed up the cellulose degradation process. These results provide comparable characterizations of the cellulolytic and hydrogen-producing capabilities of the mono-culture and co-culture systems, and the identification of ecological relationship between these two organisms will contribute to the future improvements of the hydrogen-producing efficiency using this approach.
In order to enhance cellulosic bioethanol production from thermophilic anaerobic bacteria, we also obtained the ethanol adapted strains of Clostridium thermocellum LQR1 and Thermoanaerobacter ethanolicus X514 through long term evolution. The evolved LQR1 and X514 strains were able to resistant to 5.4% and 3.9% ethanol respectively. Even though the parent strain had a greater biomass than most of the ethanol-tolerant derivatives when cultured in the absence of ethanol, the ethanol-evolved LQR1 can produce more ethanol than its ancestor. When using ethanol evolved LQR1 as the cellulose degrader, 15% more ethanol can be produced than the parent strain when co-cultured with parent X514. These results demonstrate that ethanol resistance can be developed by adaptive evolution and ethanol production can be promoted using ethanol evolved strains.
In summary, this study provided novel insights of the improvement of cellulose bioconversion process to produce biofuels, such as hydrogen and ethanol, which could be of merit for the application of Clostridia in to the industrial field and make progress with the production of second generation biofuels from the lignocellulose biomass. | en_US |
