To investigate the characteristics and the microbial diversity of denitrifying phosphate-accumulating organisms (DNPAOs) that are capable of conducting enhanced biological phosphorus removal (EBPR) using nitrate as electron acceptor, three sequencing batch reactors were operated under three different electron acceptor conditions, i.e., only oxygen, oxygen together with nitrate and only nitrate. Based on the chemical analysis concerning the biochemical transformation of each reactor, it was found that phosphate-accumulating organisms responsible for EBPR consisted of at least three populations including DNPAOs, and that the microbial community structure was changed according to the electron acceptor conditions. Also, the sludge cultivated with oxygen together with nitrate showed a drastic increase in the amount of phosphorus uptake under anoxic conditions, which suggested that a proportion of DNPAOs capable of utilizing nitrate under aerobic conditions were present. On the other hand, the change in microbial community structure depending on the type of electron acceptor was demonstrated by the analysis of the results of denaturing gradient gel electrophoresis of polymerase chain reaction-amplified 16S ribosomal DNA fragments. It was found that the bacteria commonly contained in all the reactors were Rhodocyclus sp. (96% identity) and Dechlorimonas sp. (97% identity) that belonged to the beta subclass of Proteobacteria on the basis of the analysis of the sequence excised from DGGE bands and the determination of phylogenetic affiliation. However, only the presence of Rhodocyclus sp. in all the reactors was demonstrated by fluorescent in situ hybridization analysis.
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