The sulfidogenic activity of two mesophilic sulfate reducing enrichment cultures was studied in H-2-fed membrane bioreactors. The two enrichment cultures had different origins; one of them was a mesophilic and the other a psychrotolerant mesophilic culture. The operational temperatures of the reactors were gradually changed: for one the temperature was increased from 9 to 30 degrees C and for the other it was decreased from 35 to 9 degrees C. The specific sulfidogenic activities were 21-31, 52-53 and 57-92 mmol SO42- g VSS-1 d(-1) at 9, 15 and 30-35 degrees C, respectively. The sulfate reduction rate of the SRB stabilized to a lower level after the temperature was decreased. The percent electron flow to sulfate reduction was on average 24-32, 50 and 47-69% at 9, 15 and 30-35 degrees C, respectively. The capability of mesophilic SRB to oxidize electron donor decreased as the temperature was decreased. The results indicate that starting of the reactor operation at 9 degrees C resulted in higher sulfidogenic activity at suboptimal temperatures and selective enrichment of the psychrotolerant species improved. The start-up of the reactor at 35 degrees C resulted in decreased sulfidogenic activity as the temperature was decreased. This indicates that the operational temperature of bioreactors with mesophilic SRB can be decreased to 15-20 degrees C and the sulfidogenic activity will decrease by 10-40%. Moreover, an operational temperature of 9 degrees C seems to be close to the lower limit of active sulfate reduction for the mesophilic enrichment cultures used in this study. (C) 2009 Elsevier Ltd. All rights reserved.