Effect of pretreatment severity on poplar digestion by fungal cellulases and C. thermocellum CBP: insights from deconstruction rates, cell wall properties, and rare poplar natural variants

Consolidated bioprocessing (CBP) is a promising approach to overcoming biomass recalcitrance; however, there is little evidence that effectually compares CBP organisms to fungal cellulases. In this study, poplar was processed by hydrothermal pretreatment (Stage 1) followed by biological digestion (Stage 2) by two methods: (1) fermentation using the native cellulolytic organism Clostridium thermocellum in a CBP platform or (2) enzymatic hydrolysis using free, fungal cellulases. Sugar release profiles were developed for Stage 1, Stage 2, and Stage 1+2 normalized relative to the amount of glucan plus xylan present in the raw biomass.  In addition, rare, low recalcitrant poplar natural variants were processed to determine the contribution plant natural variation might have on overcoming recalcitrance when combined with either fungal enzymes or C. thermocellum CBP. Cell wall properties including cellulose molecular weight, accessibility, and crystallinity and material composition were measured before and after Stage 1 and Stage 2 operations. For the first time, sugar release by C. thermocellum was compared to that by free, fungal cellulase cocktails at two enzyme doses, a moderate loading and a high loading (10 and 50 mg protein/g glucan, respectively) over a range of incubation times and pretreatment severity factors, log R₀, with considerations given to natural variants and changes in cell wall properties. From these results, C. thermocellum proved to be more effective in liberating sugars from hydrothermally pretreated poplar than free, fungal cellulase preparations and with more than a 10% relative increase in glucan plus xylan conversion when utilizing rare natural variants.