Microelectrode, fluorescence in situ hybridization (FISH), and denaturing gradient gel electrophoresis (DGGE) analyses were used to investigate the effect of nitrite and nitrate on in situ sulfide production in an ac activated sludge immobilized agar gel film. Microelectrode measurements of O-2, H2S, NO3-, NO2-, and pH revealed that the addition of NO2- and NO3- forced sulfate reduction zones deeper in the agar gel and significantly reduced the in situ sulfide production levels. The sulfate reduction zone was consequently separated from 02 and NO2- or NO3- respiration zones with increasing the concentrations of NO2- and NO3-. These NO2- and NO3- treatments had only a transient effect on sulfide production. The in situ sulfide production quickly recovered to the previous levels when NO2- and NO3- were removed. The PCR-DGGE and FISH analyses revealed that 2-day-continuous addition of 500 muM NO3- did not change the metabolically active sulfate-reducing bacterial (SRB) community. On the basis of these data, it could be concluded that the addition of NO2- and NO3- did not kill SRB, but induced the interspecies competition for common carbon source (i.e., acetate) between nitrate-reducing heterotrophic bacteria and SRB and enhanced the oxidation of the produced sulfide, which were main possible causes of the suppression of in situ sulfide production in the agar gel. (C) 2003 Wiley Periodicals, Inc.