Integrated genomics provides insights for the evolution of the polyphosphate accumulation trait of Ca. Accumulibacter

Abstract

Candidatus Accumulibacter plays a major role in enhanced biological phosphorus removal (EBPR), but the key genomic elements in metagenome assembled genomes enabling their phosphorus cycling ability remain unclear. Pangenome analyses were performed to systematically compare the genomic makeup of Ca. Accumulibacter and non-Ca. Accumulibacter members within the Rhodocyclaceae family. Metatranscriptomic analyses of an enrichment culture of Ca. Accumulibacter clade IIC strain SCUT-2 were performed to investigate gene transcription characteristics in a typical anaerobic-aerobic cycle. Two hundred ninety-eight core genes were shown to be obtained by Ca. Accumulibacter at their least common ancestor. One hundred twenty-four of them were acquired via horizontal gene transfer (HGT) based on best-match analysis against the NCBI database. Fourty-four laterally derived genes were actively transcribed in a typical EBPR cycle, including the polyphosphate kinase 2 (PPK2) gene. Genes in the phosphate regulon (Pho) were poorly transcribed. Via a systematical analysis of the occurrences of these genes in closely related Dechloromonas-polyphosphate accumulating organisms (PAOs) and Propionivibrio-non-PAOs, a Pho dysregulation hypothesis is proposed to explain the mechanism of EBPR. It states that the PhoU acquired by HGT fails in regulating the high-affinity phosphate transport (Pst) system. To avoid phosphate poisoning, the laterally acquired PPK2 is employed to condense excess phosphate into polyphosphate. Alternatively, genes encoding PhoU and PPK2 are obtained from different donor bacteria, leading to unmatched phosphate concentration thresholds for their activation/inactivation. PPK2 tends to reduce the intracellular phosphate to concentration levels perceived by PhoU as low-phosphate states. PhoU is not activated to turn off the Pst system, resulting in continuous phosphate uptake. In conclusion, based on integrated genomic analyses, the HGT of phoU and ppk2 and the resultant Pho dysregulation may have triggered the development and evolution of the P cycling trait in Ca. Accumulibacter.