Overflow metabolism and growth cessation in Clostridium thermocellum DSM1313 during high cellulose loading fermentations

5 years ago

Overflow metabolism and growth cessation in Clostridium thermocellum DSM1313 during high cellulose loading fermentations

Cong T. Trinh, R. Adam Thompson

As a model thermophilic bacterium for the production of second-generation biofuels, the metabolism of Clostridium thermocellum has been widely studied. However, most studies have characterized C. thermocellum metabolism for growth at relatively low substrate concentrations. This outlook is not industrially relevant, however, as commercial viability requires substrate loadings of at least 100 g/L cellulosic materials. Recently, a wild-type C. thermocellum DSM1313 was cultured on high cellulose loading batch fermentations and reported to produce a wide range of fermentative products not seen at lower substrate concentrations, opening the door for a more in-depth analysis of how this organism will behave in industrially relevant conditions. In this work, we elucidated the interconnectedness of overflow metabolism and growth cessation in C. thermocellum during high cellulose loading batch fermentations (100 g/L). Metabolic flux and thermodynamic analyses suggested that hydrogen and formate accumulation perturbed the complex redox metabolism and limited conversion of pyruvate to acetyl-CoA conversion, likely leading to overflow metabolism and growth cessation in C. thermocellum. Pyruvate formate lyase (PFL) acts as an important redox valve and its flux is inhibited by formate accumulation. Finally, we demonstrated that manipulation of fermentation conditions to alleviate hydrogen accumulation could dramatically alter the fate of pyruvate, providing valuable insight into process design for enhanced C. thermocellum production of chemicals and biofuels. Biotechnol. Bioeng. 2017;9999: 1–13. © 2017 Wiley Periodicals, Inc. Clostridium thermocellum is highly effective at utilizing cellulosic materials, but its complex metabolism has limited yields of target chemicals. In this study, genome-scale metabolic flux and thermodynamic analyses were performed to elucidate key metabolic processes that caused growth cessation and overflow metabolism in C. thermocellum during high cellulose loading fermentations. Based on model prediction, fermentation conditions were perturbed to alternate overflow metabolism. Findings provide metabolic engineering and fermentation strategies to achieve high product yields for high substrate loading fermentations.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/bit.26374