Fermentation of Dilute Acid Pretreated Populus by Clostridium thermocellum, Caldicellulosiruptor bescii, and Caldicellulosiruptor obsidiansis

Consolidated bioprocessing (CBP), which merges enzyme production, biomass hydrolysis, and fermentation into a single step, has the potential to become an efficient and economic strategy for the bioconversion of lignocellulosic feedstocks to transportation fuels or chemicals. In this study, we evaluated wild-type Clostridium thermocellum, Caldicellulosiruptor bescii, and Caldicellulosiruptor obsidiansis, three thermophilic, cellulolytic, mixed-acid fermenting candidate CBP microorganisms, for their fermentation capabilities using dilute acid pretreated Populus as a model biomass feedstock. Under pH-controlled anaerobic fermentation conditions, each candidate successfully digested a minimum of 75 % of the cellulose from dilute acid pretreated Populus, as indicated by an increase in planktonic cells and end-product metabolites and a concurrent decrease in glucan content. C. thermocellum, which employs a cellulosomal approach to biomass degradation, required approximately 50 h to achieve 75 % cellulose utilization. In contrast, the noncellulosomal, secreted hydrolytic enzyme system of the Caldicellulosiruptor sp. required about 100 h after a significant lag phase to achieve similar results. End-point fermentation conversions for C. thermocellum, C. bescii, and C. obsidiansis were determined to be 0.29, 0.34, and 0.38 g of total metabolites per gram of loaded glucan, respectively. These data provide a starting point for future strain engineering efforts that can serve to improve the biomass fermentation capabilities of these three promising candidate CBP platforms.