The temperature effect (37-65 degrees C) on H(2) production from glucose in an open-mixed culture bioreactor using an enrichment culture from a hot spring was studied. The dynamics of microbial communities was investigated by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). At 45 and 60 degrees C the H(2) production was the highest i.e. 1.71 and 0.85 mol H(2)/mol glucose, respectively. No H(2) was produced at temperatures 50 and 55 degrees C. At 37-45 degrees C, H(2) production was produced by butyrate type fermentation while fermentation mechanism changed to ethanol type at 60 degrees C. Clostridium species were dominant at 37-45 degrees C while at 50-55 degrees C and 60 degrees C the culture was dominated by Bacillus coagulans and Thermoanaerobacterium, respectively. In the presence of B. Coagulans the metabolism was directed to lactate production. The results show that the mixed culture had two optima for H(2) production and that the microbial communities and metabolic patterns promptly changed according to changing temperatures. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.