Electron flow boosted highly selective ammonium production from microbial nitrate reduction

Microbial dissimilatory nitrate reduction to ammonium (DNRA) process is considered as a bridge connecting nitrification and denitrification processes, which helps to improve the removal efficiency of nitrate wastewater. However, the competitiveness of DNRA was weaker than the denitrification, resulting in over 60-70% of nitrogen loss each year as N₂ or N₂O via denitrification. Here, we report a strategy of employing electron flow to rapidly initiate DNRA process (e-DNRA) with no external energy input. This e-DNRA strategy lies in establishing a high carbon-to-nitrogen ratio habitat to create favorable conditions for the growth and reproduction of DNRA bacteria. Subsequently, DNRA functional bacteria are enriched under reducing microenvironment induced by electron flow, ultimately forming a stable biofilm with high DNRA activity. By controlling extracellular electron flow, a nitrate reduction efficiency, conversion efficiency of nitrate to ammonium, and yield rate of 93.2%, 92.7%, and 1.23 µmmol N d^-1 g^-1 (MLSS) m^-3 were achieved, respectively. Transcriptome analysis and ¹⁵N isotope tracing technology demonstrated that electron flow promoted the expression of nrfA gene by an order of magnitude. Genus-level microbial community structure revealed species Lentimicrobium, Geobacter, and Thauera are the primary determinants for the high DNRA efficiency. Moreover, metagenome-assembled genomes found that the electron flow increased the expression of cyt b and cyt c1 subunits in complex III by 1-2 orders of magnitude which sustained the high-rate DNRA. The proposed e-DNRA strategy provides a new solution for the synergistic treatment of nitrate wastewater and ammonium recovery.