In this study, microbial responses of a continuous hydrogen reactor fed with galactose have been investigated. Process disturbances reduced H-2 production performance as well as large fluctuations in microbial diversity. The peak values of the hydrogen yield (HY) was not influenced greatly during the steady state period, and accounted as 2.01 +/- 0.05 and 2.14 +/- 0.03 mol/mol galactose(added), while hydraulic retention time (HRT) was at 12 and 8 h, respectively. Microbial community analysis via 454 pyrosequencing revealed that functional redundancy following changes in the microbial community distribution led to the stability of the fermentation performance. The butyrate to acetate (B/A) ratio well correlated with changes in the microbial community. The energy generation rate and energy yield resulted in the peak values of 134 kJ/L-d and 612 kJ/mol(added). (C) 2016, The Society for Biotechnology, Japan. All rights reserved.