The diversity of autotrophic ammonia-oxidizing bacteria (AOB) of the b-subdivision of the class Proteobacteria was investigated in a laboratory-scale denitrification-nitrification bioreactor (DNB) treating a synthetic waste stream. 16S ribosomal RNA (rRNA) gene sequences were amplified from DNA extracted from the oxic DNB sludge. Comparative analysis of the rRNA sequences revealed considerable diversity among the AOB-like sequences. The majority of sequences recovered were related to Nitrosomonas spp. but a smaller number of Nitrosospira-like sequences were obtained. Since different AOB may have different kinetic properties the high diversity of AOB, even in a simple laboratory biotreater treating a simple waste stream, has important implications for the operation of nitrifying wastewater treatment processes.

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