Denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA) Activities in Freshwater Sludge and Biofloc from Nile Tilapia Aquaculture Systems

Abstract

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 NO3 –-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 NO3 –-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.