# Wastewater Treatment with Bacterial Representatives of the _Thiothrix_ Morphotype
## 3 Methods for Identification of _Thiothrix_ in Wastewater Treatment Plants
### Nitrogen Removal  
In addition to phosphorus, nitrogen is one of the two major biogenic elements that threaten to upset the equilibrium in water bodies under anthropogenic impacts. While phosphorus and especially phosphates, which pose the greatest problem for water treatment, are mainly responsible for ecological damage to natural inland water reservoirs, nitrogenous compounds are more problematic for marine water reservoirs, although both types of compounds are dangerous for any water body under uncontrolled inputs. Members of the _Thiothrix_ morphotype are prominent representatives of sulfur-oxidizing bacteria with metabolic potential to remove toxic nitrates from wastewater. For many species of the genus _Thiothrix_, the ability to denittiry during mixotrophic growth in the presence of reduced sulfur and organic compounds (lactate and acetate) has been experimentally demonstrated [105]. No data are available on the ability of _Thiothrix_ pure cultures to denittiry in the absence of reduced sulfur compounds. An analysis of the Pangenome of the genus _Thiothrix_ showed the presence of denitrification genes in the variable part of the Pangenome. All members of the genus, with the exception of _Ca_. Thiothrix annitrata' and _'Ca_. Thiothrix sulfatifontis' KT, have genes encoding at least one form of nitrate reductase, which reduces nitrate to nitrite (Table 1). Most strains contain genes for nitrite reductase (_nirS_), which reduces nitrite to nitric oxide, and nitric oxide reductase NorBC, which reduces nitric oxide to nitrous oxide. The nitrous oxide reductase NosZ genes were found in only one of the strains involved in the Pangenomic analysis [4]. In case of uncontrolled inputs due to effluents from animal farms, agricultural factories, industrial companies and the sewage systems of residential complexes, high concentrations of nitrogen compounds lead to the destructive eutrophication of water bodies and ammonium toxicity to fish and other aquatic fauna as well as flora. [PERSON] et al. presented traditional schemes for the biological treatment of wastewater contaminated with nitrogenous compounds [116]. These schemes are based on the use of specialized bacteria with different types of nitrogen metabolism, which allows the sequential conversion of toxic ammonium ions (NH\({}_{4}\)+) to safe dinitrogen molecules (N\({}_{2}\)) through a chain consisting of nitrifier bacteria, which oxidize ammonium (AOB) and nitrite (NOB), and denittifier bacteria, which reduce nitrate at the expense of organic carbon, which acts as an electron donor (Figure 7). In this scheme, representatives of the _Thiothrix_ morphotype can play a role in the last stage. Representatives of the _Thiothrix_ morphotype are often found in nitrification-denitrification systems [32; 117; 118; 119; 120]. Their role in these systems can involve denitrification coupled with the oxidation of reduced sulfur compounds. In this case, sulfides can be used as an electron donor for the reduction of nitrates and nitrites [121]. There are reports that the _Thiothrix_ morphotype is capable of nitrate-mediated sulfide oxidation [46]. It was shown that bacteria similar in morphotype to _Thiothrix_ began to play the role of denittifiers in the simultaneous nitrification-denitrification system when the oxygen concentration decreased [122].