Expanding the diversity of Accumulibacter with a novel type and deciphering the transcriptional and morphological features among co-occurring strains

Accumulibacter is the major polyphosphate-accumulating organism (PAO) in global wastewater treatment systems. Its phylogenetic and functional diversity has been continuously updated in recent years. In addition to its widely recognized two sublineages, Types I and II, here we discovered a novel type enriched in laboratory bioreactors. Core gene- and machine learning-based gene feature profiling supported that Type III Accumulibacter was potential PAO with the unique function of using dimethyl sulfoxide as electron acceptor. On the basis of the correlation between the similarity of ppk1 and genome, the number of ppk1-represented Accumulibacter species was estimated to be over one hundred, suggesting that the currently recognized species are only the tip of the iceberg. Meanwhile, multiple Accumulibacter strains co-occurring in a bioreactor were investigated for their inter-strain transcriptional and morphological features. Metatranscriptomics of seven co-occurring strains indicated that the expression level and interphasic dynamics of PAO phenotype-related genes had minimal correlation with their phylogeny. In particular, expression of denitrifying and poly-P metabolism genes had higher inter-strain and interphasic divergence compared with glycogen and polyhydroxyalkanoates metabolic genes. A strategy of cloning rRNA genes from different strains based on similar genomic synteny was successfully applied to differentiate their morphology via fluorescence in situ hybridization. Our study further expanded the phylogenetic and functional diversity of Accumulibacter. We also proposed that deciphering the function and capability of certain Accumulibacter should be environment- and population-specific. Importance Accumulibacter, as the core functional but uncultured taxa for enhanced biological phosphorus removal, has attracted much attentions on its phylogenetic and functional diversity and intra-genus niche differentiation in the last two decades. It was well-known that this genus had two sub-lineages (Type I and II) since 2002. In this study, a novel type (Type III) with proposed novel functional feature was discovered by the metagenomic approach. By linking average nucleotide identity of Accumulibacter genomes and the similarity of the ppk1 sequences, a phylogenetic biomarker that has been largely deposited in database, we estimated that its global species-level diversity was higher than 100. Moreover, as we found the co-occurrence of multiple Accumulibacter strains in one bioreactor, the simultaneous transcriptional divergence of the co-occurring strains was interesting for understanding their niche differentiation in a single community. The results suggested the decoupling feature between transcriptional pattern with phylogeny for co-occurring strains.