Expanding the Diversity of Accumulibacter with a Novel Type and Deciphering the Transcriptional and Morphological Features among Co-Occurring Strains.

"Candidatus Accumulibacter" is the major polyphosphate-accumulating organism (PAO) in global wastewater treatment systems, and its phylogenetic and functional diversity have expanded in recent years. In addition to the widely recognized type I and II sublineages, we discovered a novel type enriched in laboratory bioreactors. Core gene and machine learning-based gene feature profiling supported the assertion that type III "Ca. Accumulibacter" is a potential PAO with the unique function of using dimethyl sulfoxide as an electron acceptor. Based on the correlation between ppk1 and genome similarity, the species-level richness of Accumulibacter was estimated to be over 100, suggesting that the currently recognized species are only the tip of the iceberg. Meanwhile, the interstrain transcriptional and morphological features of multiple "Ca. Accumulibacter" strains co-occurring in a bioreactor were investigated. 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, the expression of denitrifying and polyphosphate (poly-P) metabolism genes exhibited higher interstrain and interphasic divergence than expression of glycogen and polyhydroxyalkanoate 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 expands the phylogenetic and functional diversity of "Ca. Accumulibacter" and proposes that deciphering the function and capability of certain "Ca. Accumulibacter" should be tailored to the environment and population in question. IMPORTANCE In the last 2 decades, "Ca. Accumulibacter" has garnered significant attention as the core functional but uncultured taxon for enhanced biological phosphorus removal due to its phylogenetic and functional diversity and intragenus niche differentiation. Since 2002, it has been widely known that this genus has two sublineages (type I and II). However, in this study, a metagenomic approach led to the discovery of a novel type (type III) with proposed novel functional features. By comparing the average nucleotide identity of "Ca. Accumulibacter" genomes and the similarity of ppk1, a phylogenetic biomarker largely deposited in databases, the global species-level richness of "Ca. Accumulibacter" was estimated for the first time to be over 100. Furthermore, we observed the co-occurrence of multiple "Ca. Accumulibacter" strains in a single bioreactor and found the simultaneous transcriptional divergence of these strains intriguing with regard to their niche differentiation within a single community. Our results indicated a decoupling feature between transcriptional pattern and phylogeny for co-occurring strains.