Differences in Xylan Degradation by Various Noncellulolytic Thermophilic Anaerobes and Clostridium thermocellum

Hemicellulose fractions with a predetermined distribution of xylose, xylooligomers, and xylan fractions were obtained through steam explosion of wood by the steam explosion-extraction process of BFA-Hamburg, Hamburg, Federal Republic of Germany. A differential utilization of various molecular-weight fractions by several thermophilic anaerobic bacteria was determined during their growth on the hemicellulose preparations.Clostridium thermocellum (60°C) first utilized the high-molecular-weight fractions (polymerization degree of 15 to 40 xylose units). Xylose and xylooligomers ofn = 2 to 5 accumulated whileC. thermocellum was not growing, as evident from the fermentation products formed. Whereas the xylan was hydrolyzed and the small oligoxylans were utilized after more than 100 h of incubation, xylose was not significantly utilized. In contrast to this,C. thermohydrosulfuricum (70°C) andThermoanaerobium brockii (70°C) utilized xylose first and then xylooligomers ofn = 2 to 5, but xylooligomers ofn greater than 6 were only slowly utilized.Thermoanaerobacter ethanolicus (70°C),Thermobacteroides acetoethylicus (70°C), andC. thermosaccharolyticum (60°C) utilized xylose preferentially. Xylooligomers ofn = 2 to 5 andn = 6 and greater were apparently concomitantly utilized without significant differences. In contrast toC. thermocellum , the non-cellulolytic organisms grew during xylan hydrolysis, producing ethanol, lactate, acetate, CO2 , and H2 .