Distinctive nitrogen oxides reduction capabilities of a Denitrifying Polyphosphate Accumulating Organisms (DPAOs) enrichment in a long-term nitrite fed sequencing batch reactor

Denitrifying polyphosphate accumulating organisms (DPAOs), are capable of nitrate and/or nitrite (NO2^-) reduction coupled to P uptake when subjected to alternating anaerobic/anoxic conditions. However, strong accumulation of nitrous oxide (N2O) has been previously observed in DPAO enrichments under high NO2^- concentrations. To date, denitrification capability and denitrifying P uptake rates of DPAOs using different electron acceptors after long-term exposure to elevated NO2^- concentration hasn't been examined. To address this knowledge gap, we operated a lab-scale SBR under alternating anaerobic/anoxic conditions with high nitrite feed for >2 years. Ex-situ batch assays were performed to clarify capacity for reduction of various nitrogen oxides and P uptake of the DPAOs enrichment culture. These batch assays revealed distinct nitrogen oxides reduction and denitrifying P uptake capabilities. High-throughput 16S rRNA sequencing combined with qPCR assays revealed the selection of putative DPAOs and other potential denitrifying microbial consortia responsible for N2O accumulation under high nitrite-feeding regimes.