The Extremely Thermophilic Genus Caldicellulosiruptor: Physiological and Genomic Characteristics for Complex Carbohydrate Conversion to Molecular Hydrogen

Extremely thermophilic, carbohydrate-utilizing bacteria from the genus Caldicellulosiruptor should be considered for biohydrogen production to take advantage of their broad growth substrate range and high substrate conversion efficiency. In fact, Caldicellulosiruptor species produce molecular hydrogen at yields approaching the Thauer limit of 4 mol H2/mol glucose equivalent. Caldicellulosiruptor species can utilize pentoses, hexoses, di/oligosaccharides, as well as complex polysaccharides, including crystalline cellulose. The broad appetite of these organisms relates to the natural environment of Caldicellulosiruptor, where they thrive at high temperatures (65–78 °C), utilizing the variable saccharide composition of lignocellulosic biomass as growth substrate. The ability to degrade recalcitrant plant biomass and utilize a wide variety of polysaccharides in their fermentation pathways sets Caldicellulosiruptor species apart from many other candidate biofuel-producing microorganisms. The conversion of lignocellulose to fuels in Caldicellulosiruptor is driven by an array of novel multi-domain glycoside hydrolases that work synergistically to degrade plant polysaccharides into oligo/monosaccharides that enter the cytoplasm via an array of carbohydrate specific ABC sugar transporters. These carbohydrates are then processed through a series of catabolic pathways, after which they enter the EMP pathway to produce reducing equivalents in the form of NADH and Fdred. The reducing equivalents are ultimately utilized by both cytoplasmic and membrane-bound hydrogenases to form molecular hydrogen. Recently completed genome sequences for a number of Caldicellulosiruptor species have revealed important details concerning how plant biomass is deconstructed enzymatically and shown significant diversity within the genus with respect to lignocellulose conversion strategies.