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The production of volatile industrial chemicals utilizing metabolically engineered extreme thermophiles offers the potential for processes with simultaneous fermentation and product separation. An excellent target chemical for such a process is acetone (T_b = 56°C), ideally produced from lignocellulosic biomass.Caldicellulosiruptor bescii(T_opt78°C), an extremely thermophilic fermentative bacterium naturally capable of deconstructing and fermenting lignocellulose, was metabolically engineered to produce acetone. When the acetone pathway construct was integrated into a parent strain containing the bifunctional alcohol dehydrogenase fromClostridium thermocellum, acetone was produced at 9.1 mM (0.53 g/L), in addition to minimal ethanol 3.3 mM (0.15 g/L), along with net acetate consumption. This demonstrates thatC. besciican be engineered with balanced pathways in which renewable carbohydrate sources are converted to useful metabolites, primarily acetone and H₂, without net production of its native fermentation products, acetate and lactate.

The extreme thermophileCaldicellulosiruptor besciisolubilizes 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, byC. besciitypically results in less than 40% carbohydrate solubilization. Mild alkali pretreatment (up to 0.09 g NaOH/g biomass) improved switchgrass carbohydrate solubilization byC. besciito 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. besciigrown 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 byC. besciiand other lignocellulolytic microorganisms.