Continuous Cellulosic Bioethanol Fermentation by Cyclic Fed-Batch Cocultivation

Abstract

f Cocultivation of cellulolytic and saccharolytic microbial populations is a promising strategy to improve bioethanol production from the fermentation of recalcitrant cellulosic materials. Earlier studies have demonstrated the effectiveness of cocultivation in enhancing ethanolic fermentation of cellulose in batch fermentation. To further enhance process efficiency, a semicontinuous cyclic fed-batch fermentor configuration was evaluated for its potential in enhancing the efficiency of cellulose fermentation using cocultivation. Cocultures of cellulolytic Clostridium thermocellum LQRI and saccharolytic Thermoanaerobacter pseudethanolicus strain X514 were tested in the semicontinuous fermentor as a model system. Initial cellulose concentration and pH were identified as the key process parameters controlling cellulose fermentation performance in the fixed-volume cyclic fed-batch coculture system. At an initial cellulose concentration of 40 g liter ؊1 , the concentration of ethanol produced with pH control was 4.5-fold higher than that without pH control. It was also found that efficient cellulosic bioethanol production by cocultivation was sustained in the semicontinuous configuration, with bioethanol production reaching 474 mM in 96 h with an initial cellulose concentration of 80 g liter ؊1 and pH controlled at 6.5 to 6.8. These results suggested the advantages of the cyclic fed-batch process for cellulosic bioethanol fermentation by the cocultures. Bioethanol remains an important renewable energy alternative to petroleum-based liquid transportation fuels (1). While bioethanol derived from food crops such as corn and sugarcane has dominated the current biofuel market, recent efforts have focused on the conversion of lignocellulosic biomass to bioethanol, i.e., cellulosic bioethanol, which is considered to be socioeconomically and environmentally more sustainable (2, 3).However, the recalcitrance of cellulosic feedstock to bioconversion has posed a major challenge to the development of effective processes for cellulosic bioethanol, which typically include separate steps of enzymatic cellulose hydrolysis and microbial ethanologenic fermentation. One strategy to improve the cellulose utilization efficiency and then the ethanol production rate is the development of microbial consortia capable of simultaneously carrying out both cellulose hydrolysis and ethanologenic fermentation, representing an implementation of the consolidated bioprocessing (CBP) concept (4). Indeed, it has been demonstrated in earlier studies that cocultivation of cellulolytic and saccharolytic microbial populations could be successfully developed in batch cultures to improve cellulose utilization and ethanol production (5-7). Subsequent studies further identified metabolic mutualism, such as the supply of growth factors and utilization of excess metabolites, as the mechanism contributing to these improvements in ethanolic cellulose fermentation by cocultivation (5,8,9), further supporting the potential of cocultivation for enhancing cellulosic bi...

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