Caldicellulosiruptor owensensis strain OLT (DSM 13100) is an obligately anaerobic, extreme thermophilic bacterium that is capable of utilizing a broad range of carbohydrates and producing H-2 as a metabolic by-product. The performance of C. owensensis on glucose and xylose was analyzed in lab-scale bioreactors to assess its potential use in biohydrogen production. Acetate, H-2, and CO2 were the main end products during exponential growth of the organism on either sugar. Lactate production was triggered during the transition into the stationary phase and was associated with an increase in the levels of specific L-lactate dehydrogenase activity. in addition, minor amounts of ethanol and propionate could be detected. H-2 and acetate yields were lower on xylose than on glucose, marking an opposite trend to biomass and lactate yields. The influence of elevated H-2 partial pressure on product distribution was more dramatic in xylose-fermenting cultures. Replacement of yeast extract in the medium with a standard vitamins solution improved H-2 yield on both sugars, where it reached 100% of the theoretical maximum, i.e. 4 mol per mol hexose, on glucose. By using the defined medium, both the maximum specific growth rate and the maximum volumetric H-2 production rate of C. owensensis increased significantly on glucose and almost doubled on xylose. Screening other sugars besides glucose and xylose revealed a clear sugar-dependent product-distribution pattern and a direct correlation between biomass and lactate yields, which might be explained considering energy metabolism of the cells. The organism is proposed as a new candidate for biohydrogen production at high yields. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.