Electron flow boosted highly selective ammonium production from microbial nitrate reduction

Area of Science:

• Environmental Microbiology
• Environmental Chemistry
• Biotechnology

Background:

• Dissimilatory nitrate reduction to ammonium (DNRA) is crucial for nitrate wastewater treatment but is often outcompeted by denitrification.
• Denitrification leads to significant nitrogen loss as N2 or N2O, necessitating improved DNRA efficiency.

Purpose of the Study:

• To develop a strategy for rapidly initiating and enhancing the DNRA process using electron flow (e-DNRA).
• To investigate the mechanisms by which electron flow influences DNRA bacteria and their activity.
• To assess the efficiency of the e-DNRA strategy for nitrate removal and ammonium recovery.

Main Methods:

• Establishing a high carbon-to-nitrogen ratio habitat to favor DNRA bacteria.
• Utilizing controlled extracellular electron flow to enrich DNRA functional bacteria and form biofilms.
• Employing transcriptome analysis and 15N isotope tracing to study gene expression and nitrogen transformation.
• Analyzing microbial community structure and performing metagenome-assembled genome analysis.

Main Results:

• Achieved high nitrate reduction (93.2%), nitrate-to-ammonium conversion (92.7%), and yield rates (1.23 µmmol N d-1 g-1 MLSS m-3).
• Electron flow significantly upregulated the expression of the nrfA gene by an order of magnitude.
• Identified Lentimicrobium, Geobacter, and Thauera as key genera driving high DNRA efficiency.
• Observed a 1-2 order of magnitude increase in the expression of complex III subunits (cyt b and cyt c1) under electron flow.

Conclusions:

• The e-DNRA strategy effectively initiates and enhances DNRA activity without external energy input.
• Electron flow promotes DNRA by increasing nrfA gene expression and enhancing respiratory complex activity.
• This approach offers a novel solution for synergistic nitrate wastewater treatment and ammonium recovery.