Effects of substrate C/N ratio on the spatial distributions of ammonia-oxidizing bacterial population and their activities in biofilms were investigated by fluorescent in situ hybridization (FISH) with 16S rRNA-targeted oligonucleotide probes and the use of microelectrodes. The increase in substrate C/N ratio (i.e., addition of acetate) immediately intensified the interspecies competition for oxygen between ammonia-oxidizing bacteria and heterotrophic bacteria, and consequently resulted in deterioration of ammonium oxidation rate especially in the surface of the biofilm. This decrease in the ammonium oxidation rate in the surface could explain a steep spatial gradient of ammonia-oxidizing bacterial population within the biofilms cultured at high substrate C/N ratios.
Furthermore, development of the spatial distributions of ammonia-oxidizing bacteria and their activities within the biofilm was followed during biofilm development. The active ammonium oxidation zone gradually shifted from the upper layer to the inner layer of the biofilm, which corresponded with the development of the spatial distributions of ammonia-oxidizing bacteria within the biofilm. Based on in situ analyses of microbial structure and activity, we discussed on mechanisms of population dynamics of ammonia-oxidizing bacteria in biofilms.
