Biotechnol Bioeng 2019 Apr 14. Epub 2019 Apr 14.
Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina.
The extreme thermophile Caldicellulosiruptor bescii solubilizes and metabolizes the carbohydrate content of lignocellulose, a process that ultimately ceases because of biomass recalcitrance, accumulation of fermentation products, inhibition by lignin moieties, and reduction of metabolic activity. Deconstruction of low loadings of lignocellulose (5 g/L), either natural or transgenic, whether unpretreated or subjected to hydrothermal processing, by C. bescii typically results in less than 40% carbohydrate solubilization. Mild alkali pretreatment (up to 0.09 g NaOH/g biomass) improved switchgrass carbohydrate solubilization by C. bescii to over 70% compared to less than 30% for no pretreatment, with two-thirds of the carbohydrate content in the treated switchgrass converted to acetate and lactate. C. bescii grown on high loadings of unpretreated switchgrass (50 g/L) retained in a pH-controlled bioreactor slowly purged (τ = 80 hr) with growth media without a carbon source improved carbohydrate solubilization to over 40% compared to batch culture at 29%. But more significant was the doubling of solubilized carbohydrate conversion to fermentation products, which increased from 40% in batch to over 80% in the purged system, an improvement attributed to maintaining the bioreactor culture in a metabolically active state. This strategy should be considered for optimizing solubilization and conversion of lignocellulose by C. bescii and other lignocellulolytic microorganisms.