Distinct Microbial Community Performing Dissimilatory Nitrate Reduction to Ammonium (DNRA) in a High C/NO₃^- Reactor

A dissimilatory nitrate reduction to ammonium (DNRA) microbial community was developed under a high organic carbon to nitrate (C/NO₃^-) ratio in an anoxic semi-continuous sequencing batch reactor (SBR) fed with glucose as the source of carbon and NO₃^- as the electron acceptor. Activated sludge collected from a municipal wastewater treatment plant with good denitrification efficiency was used as the inoculum to start the system. The aim of this study was to examine the microbial populations in a high C/NO₃^- ecosystem for potential DNRA microorganisms, which are the microbial group with the ability to reduce NO₃^- to ammonium (NH₄⁺). A low C/NO₃^- reactor was operated in parallel for direct comparisons of the microbial communities that developed under different C/NO₃^- values. The occurrence of DNRA in the high C/NO₃^- SBR was evidenced by stable isotope-labeled nitrate and nitrite (¹⁵NO₃^- and ¹⁵NO₂^-), which proved the formation of NH₄⁺ from dissimilatory NO₃^-/NO₂^- reduction, in which both nitrogen oxides induced DNRA activity in a similar manner. An analysis of sludge samples with Illumina MiSeq 16S rRNA sequencing showed that the predominant microorganisms in the high C/NO₃^- SBR were related to Sulfurospirillum and the family Lachnospiraceae, which were barely present in the low C/NO₃^- system. A comparison of the populations and activities of the two reactors indicated that these major taxa play important roles as DNRA microorganisms under the high C/NO₃^- condition. Additionally, a beta-diversity analysis revealed distinct microbial compositions between the low and high C/NO₃^- SBRs, which reflected the activities observed in the two systems.