Anaerobic microflora enriched for dark fermentative H-2 production from a mixture of glucose and xylose was used in batch cultivations to determine the effects of sudden short-term temperature fluctuations on H-2 yield and microbial community composition. Batch cultures initially cultivated at 55 degrees C (control) were subjected to downward (from 55 degrees C to 35 degrees C or 45 degrees C) or upward (from 55 degrees C to 65 degrees C or 75 degrees C) temperature shifts for 48 h after which, each culture was transferred to a fresh medium and cultivated again at 55 degrees C for two consecutive batch cycles. The average H(2)( )yield obtained during the first cultivation at 55 degrees C was 2.1 +/- 0.14 mol H-2 Mol(-1) hexose equivalent. During the temperature shifts, the obtained H-2 yields were 1.8 +/- 0.15, 1.6 +/- 0.27 and 1.9 +/- 0.00 mol H-2 mol(-1) hexose equivalent at 35 degrees C, 45 degrees C and 65 degrees C, respectively, while no metabolic activity was observed at 75 degrees C. The sugars were completely utilized during the 48 h temperature shift to 35 degrees C but not at 65 degrees C and 45 degrees C. At the end of the second cycle after the different temperature shifts, the H-2 yield obtained was 96.5, 91.6, 79.9 and 54.1% (second cycle after temperature shift to 35 degrees C, 45 degrees C, 65 degrees C and 75 degrees C, respectively) when compared to the average H-2 yield produced in the control at 55 degrees C. Characterization of the microbial communities present in the control culture at 55 degrees C showed the predominance of Thermoanaerobacteriales, Clostridiales and Bacilliales. The microbial community composition differed based on the fluctuation ternperature with Thermoanaerobacteriales being most dominant during the upward temperature fluctuations and Clostridiales being the most dominant during the downward temperature fluctuations. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.