Effect of high electron donor supply on dissimilatory nitrate reduction pathways in a bioreactor for nitrate removal

• High C/NO3− ratios and/or sulfide concentrations do not necessarily lead to a shift from DEN to DNRA.
• Microbial communities do not always use the energetically most favorable process.
• Reaction rates and biomass build-up are as important as the theoretical energy gain.

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.