# Wastewater Treatment with Bacterial Representatives of the _Thiothrix_ Morphotype
## 3 Methods for Identification of _Thiothrix_ in Wastewater Treatment Plants
### Sulfate Removal  
As prominent representatives of the group of sulfur-oxidizing bacteria, members of the _Thiod thrix_ morphotype have the metabolic potential to remove toxic sulfide from wastewater. The incoming raw sewage, which contains sulfide resulting from the degradation of protein substrates, is the most probable source of _Thiod thrix_ found in activated sludge from wastewater treatment plants. For example, the same _Thiod thrix_ phylotype was detected both in influent raw sewage and in activated sludge from Moscow wastewater treatment plants, and its percentage in the influent microbial community was much higher than in activated sludge and treated water (0.79% vs. 0.05% vs. 0.17%); for the list of 16S rRNA gene reads, see Table S1, SM in Ref. [82]. A well-developed system for sulfur metabolism underlies the broad capabilities of representatives of the genus _Thiod thrix_ to participate in wastewater treatment from sulfur compounds. Genome studies of all known _Thiod thrix_ species indicate that all of them contain genes for dissimilatory sulfur metabolism (Figure 3a). The pangenome of the genus _Thiod thrix_ comprises _sqr_ (_sqrF_, _sqrA_) genes encoding sulfide: quinone oxidoreductase, which is involved in the oxidation of hydrogen sulfide to elemental sulfur; _fccAB_ genes encoding sulfide-flavocytochrome-_c_ dehydrogenase (FCSD); _soxAXBYZ_ genes encoding a branched SOX system for the oxidation of thiosulfate to sulfur and sulfate; genes _dsrABEFHNEMKIJONR_ encoding the rDSR complex that carries out the oxidation of sulfur to sulfite; _soeABC_ genes encoding quinone-dependent sulfite dehydrogenase catalyzing the direct oxidation of sulfite to sulfate; genes _sat_ encoding ATPase-sulfurylase of the dissimilatory type; and _aprAB_ genes encoding APS reductase for indirect sulfite oxidation [5]. A genome analysis of representatives of other _Thiodircase_ genera, namely _Thiodinea disciformis_, _Thiodinea eikelboomii_ and _Thiodinum flexile_, showed the presence of all genes of dissimilatory sulfur metabolism with the exception of ATP-sulfurylase of the dissimilatory type (_sat_) and APS-reductase (_aprAB_) for indirect sulfite oxidation, as well as the lack of _fccB_ genes of the FCSD complex in _Thiodinum flexile_[5]. Thus, among the representatives of the _Thiothrix_ morphotype, only representatives of the genus _Thiothrix_ have the most developed system of oxidative dissimilatory sulfur metabolism (Figure 3b). This suggests that they are the most universal candidates for systems of water treatment from sulfur-containing compounds. Based on their morphology, microorganisms similar to _Thiothrix_ sp. were identified in the sludge beds of upflow anaerobic sludge blanket (UASB) reactors, where the ability of these organisms to oxidize sulfide was demonstrated [45]. When the sulfide concentrations temporarily increased in a fluidized bed reactor used to remove nitrates from a recirculating marine fish culture system, whitish tufts identified as the _Thiothrix_ morphotype rapidly developed [85]. In these systems, representatives of the _Thiothrix_ morphotype participated in sulfide oxidation using nitrate or oxygen as an electron acceptor [46]. Bacteria similar in morphotype to _Thiothrix_ were also found as part of a composite microbial biofilm for the treatment of sulfide-containing wastewater [70] and in a laboratory biodrainage filter treating high concentrations of hydrogen sulfide [74; 86]. Figure 3: Sulfur metabolism in representatives of _Thiothrix_ morphotype. (**a**) EVenn diagram presenting sulfur metabolism genes found in the genomes of the genera _Thiothrix_, _Thiophilum_ and _Thiolinea_. Panel with _Thiothrix_ genes is based on pangenome analysis involving 15 genomes of _Thiothrix_ _llorials_ AS\({}^{\rm T}\), _Thiothrix lacustris_ BL\({}^{\rm T}\), _Thiothrix fructosivorans_ Q\({}^{\rm T}\), _Ca_. Thiothrix putei’, _Thiothrix calidrontis_ G1\({}^{\rm T}\), _Thiothrix winogradskyi_ CT3\({}^{\rm J}\), _Ca_.