## 4 Discussion
### Changes in Microbial Community Composition during the Degradation of 170 mM Butyrate  
Bacteria most closely related (among cultured strains) to _Syntrophomonas wolfei_ subsp. _saponavida_ DSM 4212 (94.7% similarity of the 16S rRNA gene sequences) were detected in the community on day 20. _S. wolfei_ is capable of syntrophic butyrate degradation [49]. Approximately 30% of the cloned bacterial sequences are related to mesophilic _Syntrophacticus schinkii_ Sp3 (96.4% 16S rRNA similarity), which are capable of syntrophic acetate oxidation (SAO) [50]. The smaller part of the bacterial community consisted of microorganismsrelated to other SAOB, including thermophilic ones: 5.1% of the cloned bacterial sequences belonged to _Thermacetogenium phaeum_ DSM 12270 (97.1% 16S rRNA similarity); 1.3% to _Pseudothermotoga profunda_ AZM34c06 (97.4% 16S rRNA similarity); and 2.5% to _Tepidiphilus margaritifer_ N2-214 (99.4% 16S rRNA similarity). Approximately 38% of the sequences belonged to _Anaerobaculum hydrogeniformis_ OS1 (100% 16S rRNA similarity). _A. hydrogeniformis_ is a typical component of microbial communities in thermophilic bioreactors. It ferents sugars, amino acids, and yeast extract, producing H\({}_{2}\)[51]. Members of the phylum _Chlorofexi_, class _Anaerolineae_, contributed to 15.3% of the bacterial sequences. _Anaerolineae_ is one of the most numerous microbial groups inhabiting anaerobic bioreactors; they are often members of anaerobic syntrophic consortia [52]. In the studied community, _Methanosarcina thermophila_ TM-1 most probably carried out acetoclastic methanogenesis, although it is also able to grow on methanol, methylamines, and H\({}_{2}\)/CO\({}_{2}\)[52]. Members of the genus _Methanothrix_ are sensitive to high acetate concentrations in the medium [53]. However, our data show that these methanogens were able to survive at acetate concentrations of up to 60 mM by aggregation, the formation of a protective external matrix, and existence inside microbial granules. A similar mechanism was described for a mesophilic methanogen _Methanothrix_ (_Methanosatea_) _harundinacea_, which was able to survive at acetate concentrations of \(\geq\)100 mM [41]. Hydrogenotrophic methanogens were represented by _Methanottermbaoter thermautotrophicus_ Delta H (100% 16S rRNA similarity) and contributed up to 3.2% of archaeal sequences. Microorganisms related to the methyl-reducing methanogens _Methanomassiliicoccus huminensis_ B10 (95.7% 16S rRNA similarity) were also detected. Members of the genus _Methanomassiliicoccus_ use methanol and methylamines as substrates and exhibit obligate dependence on hydrogen or formate [54]. Obligatory acetoclastic _Methanothrix_ spp., which exhibited high affinity for acetate and can utilize it at very low concentrations [55], dominated in the original inoculum. They were replaced by _Methanosarcina_ spp. with a lower affinity for acetate but higher growth rates and an ability to use other substrates for methanogens along with acetate. Increasing acetate concentrations in the medium were accompanied by the accumulation of _Methanosarcina_-like microorganisms. When acetate concentration further increased to \(\geq\)100 mM, the share of _Methanosarcina_ in the community decreased and hydro electrophorphic _Methanottermobacter_ became the predominant methanogenic archaea. The accumulation of _Methanothermobacter_ in the community was accompanied by the increased abundance of bacteria related to SAOB. [PERSON] et al. reported similar changes in the population of methanogenic archaea: increasing organic loading rate resulted in the decrease in the share of _Methanosarcinales_ from 81.3 to 54%, while the relative abundances of _Methanobacteriales_ and _Methanomicrobiales_ increased from 5 to 17% and from 11 to 25%, respectively [52]. The shift of the key pathway of methanogenesis was the major mechanism for the adaptation of the microbial community to high VFA concentration.