Polyphosphate-accumulating organisms (PAOs) are a group of

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s that, under certain conditions, facilitate the removal of large amounts of

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

from their environments. The most studied example of this phenomenon is in polyphosphate-accumulating bacteria (PAB) found in a type of wastewater processing known as enhanced biological phosphorus removal (EBPR), however phosphate hyperaccumulation has been found to occur in other conditions such as soil and marine environments, as well as in non-bacterial organisms such as fungi and algae. PAOs accomplish this removal of phosphate by accumulating it within their cells as

polyphosphate A polyphosphate is a Salt (chemistry), salt or ester of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic (also called, ring) structure ...

. PAOs are by no means the only microbes that can accumulate phosphate within their cells and in fact, the production of polyphosphate is a widespread ability among microbes. However, PAOs have many characteristics that other organisms that accumulate polyphosphate do not have that make them amenable to use in

wastewater treatment Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

. Specifically, in the case of classical PAOs, is the ability to consume simple carbon compounds (energy source) without the presence of an external electron acceptor (such as nitrate or oxygen) by generating energy from internally stored polyphosphate and glycogen. Many bacteria cannot consume carbon without an energetically favorable electron acceptor and therefore PAOs gain a selective advantage within the mixed microbial community present in the activated sludge. Therefore, wastewater treatment plants that operate for enhanced biological phosphorus removal have an anaerobic tank (where there is no nitrate or oxygen present as external electron acceptor) prior to the other tanks to give PAOs preferential access to the simple carbon compounds in the wastewater that is influent to the plant.

Metabolisms

Classical (Canonical) PAO Metabolism

The classical or "canonical" behavior of PAOs is considered to be the release of phosphate (as

orthophosphate In chemistry, a phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners ...

) to the environment and transformation of intracellular polyphosphate reserves into

polyhydroxyalkanoates Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and as a carbon store. Mor ...

(PHA) from

volatile fatty acids Short-chain fatty acids (SCFAs) are fatty acids of two to six carbon atoms. The SCFAs' lower limit is interpreted differently, either with one, two, three or four carbon atoms. Derived from intestinal microbial fermentation of indigestible foods, ...

(VFAs) and glycogen during anoxic conditions. This is followed by the consumption of the PHA/VFAs and uptake of environmental orthophosphate during oxic conditions to regenerate polyphosphate reserves within the cell.

Non-Canonical (or "Fermentative) PAO Metabolism

Some PAOs have been found to have alternative methods to accumulating polyphosphate, particularly to do with not storing PHA or glycogen. This is generally believed to be seen more often in extracellular environments high in organic compounds, thus containing fermentable substrates like amino acids and sugars. However, the exact mechanisms of these microbes to accumulate and use polyphosphate are not well understood.

Known Bacterial PAOs

''Candidatus'' ''Phosphoribacter'' (previously referred to as '' Tetrasphaera'' prior to 2022)

''Candidatus'' ''Phosphoribacter'' is a bacterial genus that has been found to be the dominant PAO associated with wastewater treatment worldwide, and has been found to often participate more in the biological removal of phosphorus than ''Candidatus'' Accumulibacter, contrary to previous understandings. This bacteria has been found to be a non-canonical (or fermentative/"fPAO") PAO, and universally lack the genetic potential to store PHA. This genus was largely found to be capable of producing the fermentation products acetate, lactate, alanine, and succinate. Additionally, it is suggested that the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s lysine, arginine, histidine, leucine, isoleucine, valine and phenylalanine may replace the canonical purpose of PHA as an energy substrate during oxic conditions, based on genomic potential and similarity to behavior of other microbial metabolisms. Alternatively, the compound

cyanophycin Cyanophycin, also known as CGP (cyanophycin granule polypeptide) or multi-L-arginyl-poly (L-aspartic acid), is a non-protein, non-ribosomally produced amino acid polymer composed of an aspartic acid backbone and arginine side groups. Cyanophycin ...

may used as an energy substrate due to the ubiquity of cyanophycin-metabolizing enzymes encoded in the species.

''Candidatus'' ''Accumlibacter phosphatis''

''Candidatus'' ''Accumulibacter phosphatis'' is one of the most well-studied PAOs, and is responsible for the development of the classical PAO metabolic model which ''Ca.'' ''Phosphoribacter'' later contradicted. Formerly considered the most important PAO in waste treatment, the bacteria is highly abundant in wastewater treatment plants globally. It can consume a range of carbon compounds, such as acetate and propionate, under anaerobic conditions and store these compounds as

(PHA) which it consumes as a carbon and energy source for growth using oxygen or nitrate as electron acceptor. Historically, the hyperaccumulation of phosphate by ''Ca''. ''Accumulibacter'' was seen as a stress response, but currently it is suggested that this behavior may play an ecological role. In combination with Ca. ''Phosphoribacter'', these two PAOs are considered to account for 24-70% of phosphorus removed from wastewater during treatment processing.

''Candidatus'' ''dechloromonas''

''Candidatus'' ''Dechloromonas'' species phosphoritropha and phosphorivorans are PAOs with classical metabolism genotype. ''Dechloromonas'' has been found in high abundances in wastewater treatment plants across the world. The two species described here, ''Dechloromonas'' phosphoritropha and phosphorivans, are the two most abundant species in waste treatment within the genus.

''Candidatus'' ''accumulimonas'' (previously referred to as ''Candidatus'' ''Halomonas phosphatis'')

''Candidatus'' ''accumulimonas'' is a species of PAO with classical metabolism phenotype.

''Microlunatis'' ''phosphovorus''

''Microlunatis'' ''phosphovorus'' is a species of PAO with likely non-canonical PAO metabolism, however exact mechanisms have not been determined. Belonging to the same phylum as ''Ca.'' ''phosphoribacter'', these two actinobacterial organisms exhibit similar metabolisms, however ''M.'' ''phosphovorus'' has been suggested to hyperaccumulate over ten times the amount of polyphosphate per cell mass dry weight compared to ''Ca.'' ''phosphoribacter'' or proteobacterial PAOs.

''Pseudomonas'' spp.

Some unnamed species of the ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...

'' genus have been observed to exhibit PAO phenotypes.

''Paracoccus'' ''denitrificans''

''Paracoccus'' ''denitrificans'' has been observed to exhibit a non-canonical PAO phenotype.

''Quatrionicoccus australiensis''

''Quatrionicoccus australiensis'' is a bacteria isolated from activated sludge which has been found to accumulate polyphosphate and PHA, thus likely having a classical PAO phenotype.

''Malikia granosa''

'' Malikia granosa'' is a bacteria isolated from activated sludge which has been found to accumulate polyphosphate and PHA, thus likely having a classical PAO phenotype.

''Lampropedia spp.''

''Lampropedia'' species, isolated from EBPR activated sludge, have been found to accumulate polyphosphate and PHA, though not to extreme degrees.

''Candidatus Microthrix''

''Candidatus Microthrix'', identified in more than one EBPR activated sludge source, is a filamentous bacteria suspected to be responsible for phosphate removal during the bulking phase of EBPR, where other PAOs decrease in abundance.

''Gemmatimonas aurantiaca''

'' Gemmatimonas aurantiaca'' is a bacteria isolated from activated sludge that has been observed to accumulate polyphosphate granules.

References

{{DEFAULTSORT:Polyphosphate-Accumulating Organisms Biotechnology Waste treatment technology Bacteriology Phosphorus