# Wastewater Treatment with Bacterial Representatives of the _Thiothrix_ Morphotype
## 3 Methods for Identification of _Thiothrix_ in Wastewater Treatment Plants
### Enhanced Biological Removal of Phosphorus  
Phosphorus is a key element responsible for the eutrophication of the aquatic environment, which leads to a shift in the equilibrium in the hydroecosystem, subsequently resulting in the development of cyanobacteria, a sharp decrease in the concentration of dissolved oxygen in water, the accumulation of biogenic toxins, the deterioration of water quality, the death of fish and other aquatic organisms and the waterlogging of the water body. In this regard, the removal of phosphorus from wastewater is an important task in preventing the harmful effects of eutrophication in the ecological, economic and social spheres. A biological approach could be among the methods to address this problem. One of the most successful directions of biotechnology is the use of colorless sulfur bacteria of the genus _Thiothrix_ in \"enhanced biological phosphorus removal\" (EBPR) systems. Based on the data of a Pangenomic analysis, it can be assumed that the members of the genus _Thiothrix_ can behave as phosphorus-accumulating organisms with mixotrophic metabolism during phosphorus removal and, at the same time, use the oxidation of the intracellular sulfur pool as an additional energy source along with poly-\(\beta\)-hydroxy-alkanoates (PHA). A large number of such systems are currently operating successfully around the world. The idea of using biologically safe methods for phosphorus removal based on modified activated sludge systems was implemented in the late 1950s when the \"enhanced biological phosphorus removal\" (EBPR) system was developed [88]. In an EBPR treatment system, phosphate-accumulating organisms (PAOs) accumulate increased amounts of polyphosphates in their cells and thus biologically remove phosphorus from wastewater [89]. A large number of enzyme proteins are known to be involved in the biological cycle of phosphorus (Figure 4). In bacteria, the fully characterized enzymes that enable the reversible accumulation of phosphorus in the form of polyphosphate (poly-P) are polyphosphate kinases Ppk1 (coupled to ATP dephosphorylation) [90] and Ppk2 (coupled to ATP/GTP dephosphorylation) [91], encoded by the _ppk1_ and _ppk2_ genes, respectively. It is quite likely that, in addition to this enzyme, there are still unknown mechanisms for poly-P synthesis, such as the Arp complex (_arpABCDEFGH_), which catalyzes poly-P synthesis in archaea and was also shown to have distant homologs in bacteria. Bacterial enzymes that regulate poly-P degradation include exopolynophatases Ppx, Epp, GppA (_ppx, epp_ and _gppA_); 50/30-nucleotidase SurE (_surE_); poly-AMP phosphotransferase PAP (_pap_); NAD kinase NadK (_nadK_); alkaline phosphatases APases (e.g., _phox, phoD, phoA_); and polyphosphate-dependent glucokinase PpgK (_ppgK_). In addition to these enzymes, P\({}_{i}\)-transporter Pit, encoded by the _pit_ gene (having low substrate affinity), and the ABC-type P\({}_{i}\)-transporter Pst, formed by four proteins encoded by the _pstSCAB_ genes (having high substrate affinity), are used to transport phosphate molecules into the cell [92]. Closely related to the Pst complex are the two-component signaling proteins PhoR and PhoB (_phoR_ and _phoB_) and a cytoplasmic protein PhoU (_phoU_), which regulates Pi entry into the cell under conditions of phosphate deficiency. According to the hypothetical scheme proposed by [PERSON] et al. [93], the metal-binding protein PhoU performs a negative signaling function, providing the bacterial cell with a mechanism to sense the level of P\({}_{i}\) in the environment. A successfully operating EBPR system requires that a mixed microbial consortium (MMC) be exposed to cyclic anaerobic-aerobic conditions while simultaneously receiving sufficient volatile fatty acids (VFAs) so as to stimulate the metabolism necessary for EBPR activation. Due to this mode of operation, MMC is enriched in phosphorus-accumulating organisms (PAOs). The physiological group of PAOs comprises representatives of various lines, including _'Ca_. Accumulibacter', _Tetrasphaera_ (later _'Ca_.