Candidatus Accumulibacter was the first microorganism identified as polyphosphate-accumulating organism (PAO), essential for phosphorus removal from wastewater. However, the lineage is quite diverse and clearly comprises multiple species. The current taxonomic framework is flawed, with the majority of the publicly available genomes or 16S rRNA gene sequences classified as "Candidatus Accumulibacter phosphatis", despite notable phylogenetic divergence. The ppk1 marker gene allows for a finer scale differentiation into "clades", but at same time, it has created confusion in taxonomic assignments. Therefore, a comprehensive reevaluation of the phylogeny is needed to establish a common understanding of this genus, both in terms of naming and basic conserved physiological traits.
We provide a reassessment of the phylogeny of the genus Ca. Accumulibacter, using a comparison between genome-, ppk1, and 16S rRNA gene-based phylogenies from a comprehensive dataset. With this approach, six novel species, along with the well-known Ca. A. phosphatis and Ca. A. aalborgensis, were identified and new species-specific FISH probes were designed to visualize them in situ. The MiDAS4 global survey, containing samples from 667 wastewater treatment plants (WWTPs) located in 33 different countries, was used to investigate their geographical distribution and factors that may influence their abundance. Ca. Accumulibacter had a higher relative abundance in WWTPs with phosphorus removal and was globally distributed, indicating the process design as a major driver for their presence. Metabolic annotation and FISH-Raman microspectroscopy confirmed the potential for the PAO metabolism in all Ca. Accumulibacter species, with detection in situ of the typical PAO storage polymers. None of the novel species encoded genes for full denitrification, but the annotation revealed fine-scale differences in the nitrate/nitrate reduction pathways, giving some insights into niche differentiation, and explaining to some extent how they could coexist while contributing to overall phosphorus and nitrogen removal.
