| Clostridium thermocellum is a strict anaerobic Gram-positive strain, whose optimumgrowth temperature and pH are55-60°Cand approximately7.0respectively.C.thermocellum can produce an enzyme complex named cellulosome which containscellulase and xylanase. Thus C.thermocellumcan utilize cellulose directly and its main meta-bolic products are ethanol, acetic acid, lactic acid, hydrogen and carbon dioxide. The secondgeneration of fuel ethanol nowadays is mainly biomass-based. C.thermocellum can degradecellulose and produce ethanol and hydrogen, which makes it one of the potential consolidatedbioprocessing (CBP) strain. C.thermocellum can only utilize5g/L of cellulose when culturedin serum botteles, because the organic acids produced lead to the growth inhibition of thestrain. What’s more, the ethanol tolerance of the strain is only1-2%，which is a bottleneck ofthe future isdustrial applicaiton of this strain. This study enhanced the ethanol tolerance ofC.thermocellum to5%through61generations sub-culture and adaptation to exogenous etha-nol stress. The ethanolconcentration of the adapted strain was1.23g/L when cultured in theserum bottles, which enhanced by20.58%compared with the wild type. When cultured in thebioreactor with cellulose concentration of20g/L, the ethanol concentration of the adaptedstrain was5.17g/L,28.93%higher than the wild type. The higher ethanol concentration indi-cates that the adaptation process is effective.Cassava pulp is easy to spoil and give off a strong offensive odour and contaminate thelocal water supply. Cassava pulp is rich in starch and cellulose; moreover, after starch extrac-tion the particle size of the lignocellulosic fibres is small, which helps to reduce energy-andcost-consuming pretreatment procedures, such as milling and delignification. Thus, cassavapulp is employed increasingly to produce fuel ethanol.A biphasic fermentation approach wasundertaken for the production of ethanol and hydrogen from cassava pulp. The glucose gener-ated by co-culture of Clostridium thermocellum and Thermoanaerobacterium aotearoensewas13.650.45g/L, which was1.75and1.17fold greater than that produced by mono-cultures ofC. thermocellum and T. aotearoense, respectively. The accumulated glucose could be utilizedrapidly by subsequently inoculated Saccharomyces cerevisiae. An inoculum ratio of1:1, athermophilic fermentation of84h, and a pulp concentration of4%proved optimal for ethanolproduction, fermentation efficiency, and productivity. Underthese conditions, the ethanol levelreached8.830.31g/L with a fermentation efficiency of64.952.71%. Hydrogen productionof4.06mmol by the co-culture system was1.54and2.09fold greater than that produced by mono-cultures of C. thermocellum and T. aotearoense, respectively. This sequential co-cultureapproach provided a consolidated bio-processing means to produce ethanol and hydrogenfrom cassava pulp.Paper sludge is asolid waste material generated from pulping and papermaking processes,which is easy to contaminate the environment. C.thermocellumcan directly ferment papersludge without any pretreatment, which indicates that C.thermocellumis a potential ethanolo-genic strain in the future lignocellulosic ethanol by consolidated bioprocessing. The ethanolconcentration reached7.41g/L when paper sludge from kraft pulping processwas used assubstrate for C.thermocellum, which increased77.9%compared to that of commercial cellu-lose as substrate. The cellulase and xylanase activity were1.92FPU/mL and1.30U/mL re-spectively. For recycled deinking paper sludge, the fermentation was inhibited and the ethanolconcentration was only1.31g/L, the cellulase and xylanase activity were1.64FPU/mL and0.90U/mL respectively. The crude enzyme produced from C.thermocellum showed the op-timal reaction condition for cellulosic degradation was60℃and pH5.5; for the xylanase was70℃and pH7.0. |
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