Biobutanol Production from Crystalline Cellulose through Consolidated Bioprocessing

Biobutanol production directly from lignocellulose, known as consolidated bioprocessing (CBP), is expected to be much less expensive than a process where hydrolytic enzyme production, cellulose saccharification, and microbial fermentation are accomplished separately. However, few microbes possess both cellulolytic and solventogenic properties in nature. Current research aims to endow cellulolytic microorganisms with butanol-producing ability or to set up microbial consortia for CBP. This review comprehensively details current achievements attempting to confer butanol-generating ability, not only to cellulolytic Clostridium strains but also to microbial consortia, to address and overcome major challenges in butanol production from cellulose. Recent advances in improving cellulosome activities within cellulolytic Clostridium strains are also emphasized. Isobutanol and butanol can be produced from cellulose in mesophilic Clostridium cellulolyticum and Clostridium cellulovorans by introducing butanol synthesis modules. Thermophilic Clostridium thermocellum shows increased cellulose degradation efficiency. Microbial consortia including cellulolytic and solventogenic strains represent a simple and efficient method to achieve butanol production from cellulose in CBP. Multivariate modular metabolic engineering (MMME) provides a versatile tool to optimize the pathway and strain interactions. Cohesin fusion of β-glucosidase into the Clostridium cellulosome can eliminate the cellobiose inhibition effect, improving the cellulose degradation efficiency of C. thermocellum. Lytic polysaccharide monooxygenases (LPMOs) have been described as ‘cellulase boosters’. Integrating bacterial LPMOs into cellulosomes increases the levels of soluble sugars released from cellulose.