Candidatus Accumulibacter phosphatis
|Candidatus Accumulibacter phosphatis|
|Candidatus Accumulibacter phosphatis (blue cells)|
Candidatus Accumulibacter phosphatis (CAP) is an unclassified Betaproteobacteria that is a common bacterial community member of wastewater treatment plants performing Enhanced biological phosphorus removal (EBPR) and is a Polyphosphate-accumulating organisms. The role of CAP in EBPR was elucidated using culture-independent approaches such as 16S rRNA clone banks that showed that the Betaproteobacteria dominated lab-scale EBPR reactors. Further work using clone banks and Fluorescence in situ hybridization (FISH) identified a group of bacteria, closely related to Rhodocyclus as the dominant member of lab-scale communities.
There are currently no cultured isolates of CAP and so the phylogeny of CAP strains is based purely of molecular biology techniques. To date the polyphosphate kinase (ppk1) and the PHA synthase (phaC)  genes have been used to characterise CAP populations at a higher resolution that 16S rRNA. The ppk1 phylogeny is more frequently used and groups CAP into two major divisions: Type I and Type II. Each of these types has a number of clades that are given a letter designation, e.g. IA, IIA, IIB, IIC. An environmental survey of wastewater treatment plants and natural waterways in California and Wisconsin in the USA revealed that there were at least five CAP I (IA .. IE) clades and seven CAP II (IIA .. IIG) clades.
CAP is yet to be cultured, however the ability to enrich lab-scale EBPR communities with up to 80% CAP  has enabled research into its metabolism using meta-omic approaches. EBPR is generally associated with three stages: anaerobic, aerobic and settling. For CAP to dominate in EBPR reactors, they must be able to thrive under these conditions. During the anaerobic phase CAP can take up volatile fatty acids and store this simple carbon source intracellularly as polyhydroxyalkanoates (PHA). At the same time intracellular polyphosphate is degraded to form ATP, releasing phosphate into the media. During the subsequent aerobic phase PHA is used for energy production and phosphate is taken up from the media to form polyphosphate. Genomic reconstruction from an EBPR reactor enriched with CAP IIA revealed that it contains two different types of phosphate transporters, the high affinity Pst and low affinity Pit transporters and well as utilising the Embden Meyerhof (EM) glycogen degradation pathway. Furthermore the CAP IIA genome contains nitrogen and CO2 fixation genes, which indicate that CAP has adapted to environments limited in carbon and nitrogen. One interesting discrepancy between the genomic data and reactor performance data was the lack of a functional respiratory nitrate reductase gene. Previous work had shown that CAP could use nitrate as the terminal electron acceptor  however the genomic data indicated that the periplasmic nitrate reductase gene could not function in the electron transport chain as it lacked the necessary quinol reductase subunit. To resolve these issues lab-scale EBPR reactors enriched with CAP IA and CAP IIA were tested for their nitrate reduction capabilities. Interestingly, CAP IA was able to couple nitrate reduction to phosphate uptake while the genomically characterised CAP IIA could not.
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