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.

Suspended organic sludge from freshwater and biofloc Nile tilapia systems were examined for the presence of denitrifying and dissimilatory nitrate reduction to ammonium (DNRA) activities under nitrate and sulfide stimulation. Initial nitrate concentrations at 25 and 100 mg NO 3 --N/L were added to the freshwater sludge and biofloc samples to simulate low and high nitrate levels that are normally found in aquaculture systems. The results showed that freshwater sludge and biofloc both had denitrifying activity immediately after nitrate addition. However, ammonium accumulated in the biofloc reactors but not in the freshwater reactors, indicating the activity of DNRA in the high C/N biofloc particles. The influence of sulfide on nitrate reduction was also studied by adding different concentrations of sulfide along with 100 mg NO 3 --N/L. The results showed that elevated sulfide concentrations partially inhibited denitrification in the freshwater sludge and caused nitrite and ammonium accumulation, in which ammonium formation was probably responsible by DNRA activity. In sulfide-added biofloc reactors, ammonium accumulated at the same level as found in the biofloc reactors without sulfide. Therefore, DNRA bacteria residing in the biofloc aquaculture system were more likely to be heterotrophs that did not use sulfide as their electron donor.