PhD Thesis Proposal: Dhananjay Beri

Friday, September 14, 2018, 10:00am

Rm. 100 (Spanos Auditorium), Cummings Hall

Studies on the utilization of hemicellulose by thermophiles and its role in effective solubilization of lignocellulosic feedstocks

Abstract

Clostridium thermocellum is one of the most efficient cellulolytic organisms and is the leading candidate for a cellulosic biofuel process utilizing a consolidated bioprocessing (CBP) configuration. The US currently produces close to 16 billion gallons of ethanol every year, almost entirely from the starch in corn kernels. The outermost pericarp later of corn kernel is made up of lignocellulose, which proves recalcitrant to enzymatic hydrolysis and ends up as animal feed after the yeast fermentation process. Conversion of this corn fiber could increase ethanol yields by about 10 % and the process can be implemented in a ‘bolt-on’ configuration at an existing ethanol plant.

We report here that Clostridium thermocellum is able to solubilize over 90% of carbohydrate in corn fiber at low solids loading, without thermochemical pretreatment save autoclaving, whereas carbohydrate solubilization yields for controls using commercial fungal cellulase are about 50%.  The solubilization products, however, are rich in complex arabinoxylan (AX) that is not readily fermented by C. thermocellum and also not by Thermoanaerobacterium saccharolyticum and several other described hemicellulose-fermenting thermophilic bacteria.  To find an organism capable of breaking down AX and growing in coculture with C. thermocellum, inoculum from a thermophilic anaerobic digester was enriched on AX and component microbes isolated. The best performing isolate, designated LL 1360, consumed 85-90% of the AX. Structural analysis of the AX led to the identification of recalcitrant glycosidic linkages.  Five enzymes involved in hydrolyzing AX linkages were identified based on the sequence of the LL1360 genome and screening studies. Supplementation with these enzymes allowed T. thermosaccharolyticum to consume 80% of GAX whereas unsupplemented controls consumed about 50%. The enzymes were successfully expressed in T. thermosaccharolyticum to obtain strain AX1 which exhibited improved AX fermentation compared to controls.

A commercial corn fiber to ethanol conversion process would require a titer of >50g/l ethanol. Although C. thermocellum can solubilize corn fiber at >90% yield at low solid loading, fermentation data shows that the %solubilisation and the rate of cellulose utilization falls considerably with increasing solid concentrations. Studies suggest that the soluble arabinoxylan liberated from corn fiber slows down the cellulolytic machinery of C. thermocellum. This inhibition at high solids needs to be investigated and possible remedies identified, to further the development of a robust C. thermocellum strain for an industrial cellulosic ethanol process.    

Thesis Committee

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.