Polyphosphate-accumulating organisms (PAOs) are a group of microorganisms that, under certain conditions, facilitate the removal of large amounts of phosphorus 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. [1] PAOs accomplish this removal of phosphate by accumulating it within their cells as polyphosphate. 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. 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. [2] 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.
The classical or "canonical" behavior of PAOs is considered to be the release of phosphate (as orthophosphate) to the environment and transformation of intracellular polyphosphate reserves into polyhydroxyalkanoates (PHA) from volatile fatty acids (VFAs) and glycogen during anoxic conditions. [3] 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. [3]
Some PAOs have been found to have alternative methods to accumulating polyphosphate, particularly to do with not storing PHA or glycogen. [4] [5] 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. [6] However, the exact mechanisms of these microbes to accumulate and use polyphosphate are not well understood. [5]
CandidatusPhosphoribacter 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. [4] [7] [5] This bacteria has been found to be a non-canonical (or fermentative/"fPAO") PAO, and universally lack the genetic potential to store PHA. [4] [8] This genus was largely found to be capable of producing the fermentation products acetate, lactate, alanine, and succinate. [8] [9] Additionally, it is suggested that the amino acids 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. [4] Alternatively, the compound cyanophycin may used as an energy substrate due to the ubiquity of cyanophycin-metabolizing enzymes encoded in the species. [4]
CandidatusAccumulibacter 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. [10] Formerly considered the most important PAO in waste treatment, the bacteria is highly abundant in wastewater treatment plants globally. [5] [11] It can consume a range of carbon compounds, such as acetate and propionate, under anaerobic conditions and store these compounds as polyhydroxyalkanoates (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. [12] In combination with Ca. Phosphoribacter, these two PAOs are considered to account for 24-70% of phosphorus removed from wastewater during treatment processing. [7]
CandidatusDechloromonas species phosphoritropha and phosphorivorans are PAOs with classical metabolism genotype. [13] Dechloromonas has been found in high abundances in wastewater treatment plants across the world. [14] [15] [16] [5] The two species described here, Dechloromonas phosphoritropha and phosphorivans, are the two most abundant species in waste treatment within the genus. [17]
Candidatusaccumulimonas is a species of PAO with classical metabolism phenotype. [18] [19]
Microlunatisphosphovorus is a species of PAO with likely non-canonical PAO metabolism, however exact mechanisms have not been determined. [20] [21] [22] 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. [21]
Some unnamed species of the Pseudomonas genus have been observed to exhibit PAO phenotypes. [23]
Paracoccusdenitrificans has been observed to exhibit a non-canonical PAO phenotype. [23] [24]
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. [25] [1]
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. [26]
Lampropedia species, isolated from EBPR activated sludge, have been found to accumulate polyphosphate and PHA, though not to extreme degrees. [27]
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. [28]
Gemmatimonas aurantiaca is a bacteria isolated from activated sludge that has been observed to accumulate polyphosphate granules. [29]
Anammox, an abbreviation for "anaerobic ammonium oxidation", is a globally important microbial process of the nitrogen cycle that takes place in many natural environments. The bacteria mediating this process were identified in 1999, and were a great surprise for the scientific community. In the anammox reaction, nitrite and ammonium ions are converted directly into diatomic nitrogen and water.
Biological augmentation is the addition of archaea or bacterial cultures required to speed up the rate of degradation of a contaminant. Organisms that originate from contaminated areas may already be able to break down waste, but perhaps inefficiently and slowly.
The activated sludgeprocess is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It is one of several biological wastewater treatment alternatives in secondary treatment, which deals with the removal of biodegradable organic matter and suspended solids. It uses air and microorganisms to biologically oxidize organic pollutants, producing a waste sludge containing the oxidized material.
Enhanced biological phosphorus removal (EBPR) is a sewage treatment configuration applied to activated sludge systems for the removal of phosphate.
Denitrifying bacteria are a diverse group of bacteria that encompass many different phyla. This group of bacteria, together with denitrifying fungi and archaea, is capable of performing denitrification as part of the nitrogen cycle. Denitrification is performed by a variety of denitrifying bacteria that are widely distributed in soils and sediments and that use oxidized nitrogen compounds such as nitrate and nitrite in the absence of oxygen as a terminal electron acceptor. They metabolize nitrogenous compounds using various enzymes, including nitrate reductase (NAR), nitrite reductase (NIR), nitric oxide reductase (NOR) and nitrous oxide reductase (NOS), turning nitrogen oxides back to nitrogen gas or nitrous oxide.
Aerobic granular reactors (AGR) or Aerobic granular sludge (AGS) are a community of microbial organisms, typically around 0.5-3mm in diameter, that remove carbon, nitrogen, phosphorus and other pollutants in a single sludge system. It can also be used for wastewater treatments. Aerobic granular sludge is composed of bacteria, protozoa and fungi, which allows oxygen to follow in and biologically oxidize organic pollutants. AGS is a type of wastewater treatment process for sewages and/or industrial waste treatment. AGR was first discovered by UK engineers, Edward Ardern and W.T. Lockett who were researching better ways for sewage disposal. Another scientist by the name of Dr. Gilbert Fowler, who was at the University of Manchester working on an experiment based on aeration of sewage in a bottle coated with algae. Eventually, all three scientists were able to collaborate with one another to discover AGR/AGS.
Nitrospira translate into “a nitrate spiral” is a genus of bacteria within the monophyletic clade of the Nitrospirota phylum. The first member of this genus was described 1986 by Watson et al. isolated from the Gulf of Maine. The bacterium was named Nitrospira marina. Populations were initially thought to be limited to marine ecosystems, but it was later discovered to be well-suited for numerous habitats, including activated sludge of wastewater treatment systems, natural biological marine settings, water circulation biofilters in aquarium tanks, terrestrial systems, fresh and salt water ecosystems, and hot springs. Nitrospira is a ubiquitous bacterium that plays a role in the nitrogen cycle by performing nitrite oxidation in the second step of nitrification. Nitrospira live in a wide array of environments including but not limited to, drinking water systems, waste treatment plants, rice paddies, forest soils, geothermal springs, and sponge tissue. Despite being abundant in many natural and engineered ecosystems Nitrospira are difficult to culture, so most knowledge of them is from molecular and genomic data. However, due to their difficulty to be cultivated in laboratory settings, the entire genome was only sequenced in one species, Nitrospira defluvii. In addition, Nitrospira bacteria's 16S rRNA sequences are too dissimilar to use for PCR primers, thus some members go unnoticed. In addition, members of Nitrospira with the capabilities to perform complete nitrification has also been discovered and cultivated.
Thiothrix is a genus of filamentous sulfur-oxidizing bacteria, related to the genera Beggiatoa and Thioploca. They are usually Gram-negative and rod-shaped. They form ensheathed multicellular filaments that are attached at the base, and form gonidia at their free end. The apical gonidia have gliding motility. Rosettes of the filaments are not always formed but are typical. Sulfur is deposited in invaginations within the cell membrane.
Triclocarban is an antibacterial chemical once common in, but now phased out of, personal care products like soaps and lotions. It was originally developed for the medical field. Although the mode of action is unknown, TCC can be effective in fighting infections by targeting the growth of bacteria such as Staphylococcus aureus. Additional research seeks to understand its potential for causing antibacterial resistance and its effects on organismal and environmental health.
Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges. Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. There are a high number of sewage treatment processes to choose from. These can range from decentralized systems to large centralized systems involving a network of pipes and pump stations which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter from sewage, using aerobic or anaerobic biological processes. A so-called quarternary treatment step can also be added for the removal of organic micropollutants, such as pharmaceuticals. This has been implemented in full-scale for example in Sweden.
"Candidatus Accumulibacter" is an unclassified group of Betaproteobacteria that currently contains only a single member, "Candidatus Accumulibacter phosphatis". "Ca. A. phosphatis" is a common bacterial community member of wastewater treatment plants performing enhanced biological phosphorus removal and is a polyphosphate-accumulating organism. There are currently no cultured representatives, however due to the importance of "Ca. A. phosphatis" in the biotechnology industry there has been much research into the physiology of these bacteria.
Candidatus Accumulibacter phosphatis (CAP) is an unclassified type of Betaproteobacteria that is a common bacterial community member of sewage treatment and wastewater treatment plants performing enhanced biological phosphorus removal (EBPR) and is a polyphosphate-accumulating organism. The role of CAP in EBPR was elucidated using culture-independent approaches such as 16S rRNA clone banks that showed the Betaproteobacteria dominated lab-scale EBPR reactors. Further work using clone banks and fluorescence in situ hybridization identified a group of bacteria, closely related to Rhodocyclus as the dominant member of lab-scale communities.
CandidatusScalindua wagneri is a Gram-negative coccoid-shaped bacterium that was first isolated from a wastewater treatment plant. This bacterium is an obligate anaerobic chemolithotroph that undergoes anaerobic ammonium oxidation (anammox). It can be used in the wastewater treatment industry in nitrogen reactors to remove nitrogenous wastes from wastewater without contributing to fixed nitrogen loss and greenhouse gas emission.
Microlunatus phosphovorus is the type species of the bacterial genus Microlunatus. It is Gram-positive and is notable for being a polyphosphate-accumulating bacterium. It is coccus-shaped, aerobic, chemoorganotrophic.
Phycicoccus elongatus is a polyphosphate-accumulating bacterium. It is meso-diaminopimelic acid-containing, gram-positive, asporogenous oval- to rod-shaped and an aerobic chemoheterotroph.
Tetrasphaera is a bacterial genus from the family of Intrasporangiaceae.
Fermentibacteria is a bacterial phylum with candidate status. It is part of the FCB group.
Modulibacteria(Moduliflexota) is a bacterial phylum formerly known as KS3B3 or GN06. It is a candidate phylum, meaning there are no cultured representatives of this group. Members of the Modulibacteria phylum are known to cause fatal filament overgrowth (bulking) in high-rate industrial anaerobic wastewater treatment bioreactors.
Anammox is a wastewater treatment technique that removes nitrogen using anaerobic ammonium oxidation (anammox). This process is performed by anammox bacteria which are autotrophic, meaning they do not need organic carbon for their metabolism to function. Instead, the metabolism of anammox bacteria convert ammonium and nitrite into dinitrogen gas. Anammox bacteria are a wastewater treatment technique and wastewater treatment facilities are in the process of implementing anammox-based technologies to further enhance ammonia and nitrogen removal.