(645e) Modeling Cocultures of Clostridium Thermocellum and Thermoanaerobacterium Saccharolyticum for Lignocellulosic Ethanol Production

Clostridium thermocellum is a thermophilic lignocellulolytic anaerobic bacteria and promising candidate for consolidated bioprocessing (CBP) of lignocellulosic biomasses (LCBs) such as switchgrass, corn stover, and poplar to produce ethanol. Recent studies have shown that hemicellulose, released from LCB by C. thermocellum’s cellulases, inhibits cellulase activity, limiting total LCB solubilization. Therefore, introducing a hemicellulose-utilizing coculture partner (such as Thermoanaerobacterium saccharolyticum) increases solubilization of LCB. It is thought that the benefit of the coculture comes from the increased solubilization and decreased inhibition of C. thermocellum, and not any metabolic synergy. To test this assumed neutralism, a new genome-scale model (GSM) of metabolism of T. thermosaccharolyticum is reconstructed, expanding an existing, but relatively small (315 genes, 528 metabolites, 539 reactions) GSM. The new reconstruction accounts for more than 500 genes, 600 reactions, and 600 metabolites, with highly curated transport reactions as well as C5 and C6 catabolic pathways. This model is used in a multi-level optimization framework for modeling microbial communities, the OptCom framework, with another highly curated model of C. thermocellum, iCTH669, which the authors have also recently reconstructed, to elucidate possible metabolic synergies between the coculture partners. Two sets of OptCom simulations of the coculture were performed, which allow and disallow metabolic crosstalk, at fixed relative abundances of coculture members to explore metabolic interactions under different coculture compositions. These simulations are repeated under various media conditions representing different LCB feedstocks. We find that allowing metabolic crosstalk shifts the relative abundance of C. thermocellum and T. saccharolyticum in the fastest growing cocultures (though C. thermocellum is still the most abundant partner) and increases coculture growth rate. Model-predicted metabolic crosstalk between these species is through relatively small magnitude exchange of amino acids, which are usual fermentation byproducts of both species, suggesting a limited mutualism which can go unnoticed when only measuring the composition of coculture fermentation products. This limited mutualism can be used as the starting point for improving coculture synergy for improved ethanol yield in future.