Thermoanaerobacter strain AK68, isolated from a hot spring in Iceland, was investigated for hydrogen production from sugars. Strain AK68 is most closely related to Thermoanaerobacter thermohydrosulfuricus (99.9% similarity). The strain has a temperature optimum between 65 degrees C and 70 degrees C and a pH optimum between pH 7.0 and 8.0. The strain degraded the majority of substrates examined including common hexoses and disaccharides as well as xylose but not arabinose or starch. Various environmental variables were investigated to gain a deeper understanding of the factors that are of importance for hydrogen production since the strain shows inhibition at relatively low initial glucose concentrations. Factors investigated included initial glucose concentration, phosphate buffer concentration, liquid gas phase ratio, effect of various external electron acceptors, and different initial pH values. Substrate utilization was shown to be inhibited by increased partial pressure of hydrogen, as evidenced by increased utilization of glucose using electron scavenging systems and manipulation of the liquid gas phase ratio, rather than substrate or end product inhibition. (C) Copyright 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.