Fe(0)-Dissimilatory Nitrate Reduction to Ammonium for Autotrophic Recovery of Reactive Nitrogen.

Bioreduction of nitrate to value-added ammonium is a potentially sustainable strategy to recycle nutrients from wastewater. Here, we have proven the feasibility of the reduction of autotrophic nitrate to ammonium with electrons extracted from Fe(0). Using a Geobacter-dominated anodic biofilm as an inoculum, we achieved nitrate-to-ammonium efficiency up to 90 ± 3% with a nitrate reduction rate of 35 ± 1.3 mg N/d/L. An electron acceptor instead of an inoculum greatly influenced the Fe(0)-dissimilatory nitrate reduction to ammonium (DNRA), where nitrite as the electron acceptor provided an effective selective pressure to enrich Geobacter from initial 5 to 56%. The DNRA repressing denitrification was demonstrated by the reverse tendencies of upregulated nrfA and downregulated nirS gene transcription. This finding provides a new route for autotrophic nitrate removal and recycling from water, which has a broader implication on biogeochemical nitrogen and iron cycling.