# Wastewater Treatment with Bacterial Representatives of the _Thiothrix_ Morphotype
## 1 Introduction  
Representatives of filamentous bacteria, similar in morphotype to _Thiothrix_, are attaching filamentous organisms, many of which are capable of forming rosettes, accumulating elemental sulfur inside cells and spreading by gonidia [1]. In running water containing hydrogen sulfide, they form abundant whitish filamentous fouling, visible to the naked eye. In the presence of reduced sulfur compounds, representatives of the _Thiothrix_ morphotype are known for their versatile metabolism, which gives them potential for lithoautotrophic and lithoheterotrophic growth [2; 3; 4; 5]. Most commonly, they can be found in sulfide springs, deep-sea hydrothermms, irrigation systems and activated sludge from sewage treatment plants [2; 3; 4; 5]. Over the last two decades, the taxonomy of the genus _Thiothrix_ has been revised several times. In the guidelines for the identification of microorganisms that cause the bulking of activated sludge, filamentous bacteria similar in morphotype to _Thiothrix_ were classified as Type 021N, _Thiothrix_ I and _Thiothrix_ II [6; 7]. However, it is worth noting that, even at the time of the publication of these manuals, such classification was shown to be phylogenetically incorrect [8; 9]. Subsequently, [PERSON] and [PERSON] [10] proposed that _Thiothrix_ _disiformis_ and _Thiothrix_ _eikelboomii_, previously included in Type 021N, should be classified as the genus _Thiothinea_, while _Thiothrix flexilis_ should be classified as the genus _Thiothum_. Despite the fact that _Thiolinea_ and _Thiofilm_ were assigned to separate genera 6 years ago, representatives of these genera are often still attributed to the genus _Thiothrix_ in works on biological wastewater treatment [11; 12]. Moreover, even the more obsolete name Type 021N was used to refer to them for nearly two decades [13; 14; 15; 16; 17; 18]. To eliminate such confusion in taxonomy, in the presented review, we combine data on bacteria currently assigned to three genera, _Thiothrix_, _Thiofilm_ and _Thiolinea_, based on phylogenetic data, in contrast to their previous assignment to the genus _Thiothrix_ only on the basis of the morphotype (Figure 1). The importance of bacteria of the _Thiothrix_ morphotype in applied studies is due to the fact that they are often found in domestic [18; 20; 21; 22; 23] and industrial [24; 25; 26] wastewater treatment systems. Biological treatment using complex microbial communities to degrade pollutants into low-toxicity or non-toxic products is the most important method for wastewater treatment [27]. Bacteria of _Thiothrix_ morphotype are often found in such microbial communities. These bacteria have been detected in wastewater treatment plants in the pulp and paper [28; 29; 30; 31] and textile industries [32], as well as in wastewater from bisphenol A production [33]. In addition, bacteria of the _Thiothrix_ morphotype actively proliferate  
Figure 1: Phylogenetic tree of the family _Thiothrix_ based on the concatenated genomic sequences of 120 conserved marker genes (maximum likelihood method). Clade I of the genus _Thiothrix_ is marked with a square bracket in the right part of the figure. For tree rooting, the genome of _Beggitaa leptomitoformis_ D-402T (GCF_001305575.3) was used. Figure was modified from [PERSON] et al. [19] under the terms of the Creative Commons Attribution 4.0 International License. in wastewater from food industries, including the dairy [34], fish canning [35] and potato industries [36]. Bacteria of the _Thiothrix_ morphotype are often found in wastewater from pharmaceutical plants. It was shown that _Thiothrix_ was one of the dominant populations at the genus level in a decomposition reactor for the wastewater treatment of limomycin [37], diclofenac and clofibrin acid [23]; spiramycin, oxytteracycline and streptomycin [38]; and tetracycline and sulfamethoxazole [39; 40]. The role of bacteria with a morphotype similar to _Thiothrix_ in wastewater treatment systems is controversial.