Population Structure and Morphotype Analysis of Candidatus Accumulibacter Using Fluorescence In Situ Hybridization-Staining-Flow Cytometry

Candidatus Accumulibacter is the dominant polyphosphate-accumulating organism (PAO) in denitrifying phosphorus removal (DPR) systems. In order to investigate the community structure and Glade morphotypes of Candidatus Accumulibacter in DPR systems through flow cytometry (FCM), denitrifying phosphorus removal of almost 100% using nitrite and nitrate as the electron acceptor was achieved in sequencing batch reactors (SBRs). An optimal method of flow cytometry combined with fluorescence in situ hybridization and SYBR green I staining (FISH-staining-flow cytometry) was developed to quantify PAOs in DPR systems. By setting the width value of FCM, bacterial cells in a sludge sample were divided into three groups in different morphotypes, namely, coccus, coccobacillus, and bacillus. Average percentages that the three different PAO populations accounted for among total bacteria from SBR1 (SBR2) were 42% (45%), 14% (13%), and 4% (2%). FCM showed that the ratios of PAOs to total bacteria in the two reactors were 61% and 59%, and the quantitative PCR (qPCR) results indicated that IIC was the dominant Candidatus Accumulibacter Glade in both denitrifying phosphorus removal systems, reaching 50% of the total Candidatus Accumulibacter bacteria. The subdominant Glade in the reactor with nitrite as the electron acceptor was IID, accounting for 31% of the total Candidatus Accumulibacter bacteria. The FCM and qPCR results suggested that clades IIC and IID were both coccus, Glade IIF was coccobacillus, and Glade IA was bacillus. FISH analysis also indicated that PAOs were major cocci in the systems. An equivalence test of FCM-based quantification confirmed the accuracy of FISH-staining-flow cytometry, which can meet the quantitative requirements for PAOs in complex activated sludge samples. IMPORTANCE As one group of the most important functional phosphorus removal organisms, Candidatus Accumulibacter, affiliated with the Rhodocyclus group of the Betaproteobacteria, is a widely recognized and studied PAO in the field of biological wastewater treatment. The morphotypes and population structure of Glade-level Candidatus Accumulibacter were studied through novel FISH-staining-flow cytometry, which involved denitrifying phosphorus removal (DPR) achieving carbon and energy savings and simultaneous removal of N and P, thus inferring the different denitrifying phosphorus removal abilities of these clades. Additionally, based on this method, in situ quantification for specific polyphosphate-accumulating organisms (PAOs) enables a more efficient process and more accurate result. The establishment of FISH-staining-flow cytometry makes cell sorting of Glade-level noncultivated organisms available.