The possible shift of a bioreactor for NO3 removal from predominantly denitrification (DEN) to dissimilatory nitrate reduction to ammonium (DNRA) by elevated electron donor supply was investigated. By increasing the C/NO3 ratio in one of two initially identical reactors, the production of high sulfide concentrations was induced. The response of the dissimilatory NO3 reduction processes to the increased availability of organic carbon and sulfide was monitored in a batch incubation system. The expected shift from a DEN-towards a DNRA-dominated bioreactor was not observed, also not under conditions where DNRA would be thermodynamically favorable. Remarkably, the microbial community exposed to a high C/NO3 ratio and sulfide concentration did not use the most energy-gaining process. (C) 2014 Elsevier Ltd. All rights reserved.