## 3 Results and Discussion
### Taxonomic Distribution of Bacterial Communities from Oil Wells near Nurlat  
One of the blocks of microbial ecology in oil environments is the study of the composition of complex microbial communities in oil-producing mixtures. Microbes associated with production mixtures have access to both electron donors and electron acceptors [41, 42]. Researchers conclude that fermentation, methanogenesis, and sulfate reduction are common to oil reservoirs; however, other microbial groups such as iron-, manganese-, or nitrate-reducing bacteria were also detected in various oil reservoirs [8, 9, 12, 17]. Understanding the microbial nature and processes in oil reservoirs is important for the oil industry, for example, for the development of technologies such as MEOR, mitigation of MIC, and bioremediation of oil-polluted sites [43, 44]. It should be noted that many species cannot be cultivated, so methods of molecular biology must be used in such studies. Five produced water samples from wells near Nurlat (PW_N1, PW_N2, PW_N3, PW_N4, and PW_N5) were characterized by a very high salinity. The pH values of all samples were in the range of 5.8-6.2, and the total solids content (as an indicator of all inorganic and organic compounds) of the water samples was in the range of 17-24%. Such produced waters contain a high level of chemical compounds, and the pH values of various oil reservoirs range from 3 to 7 [18, 45, 46]. Given that analysis of the water and oil phases revealed huge differences in the composition of their microbial communities [41, 43], we did not separate the oil-water mixture to obtain a general understanding of the composition of the microbial community. Total DNA was isolated from enrichments obtained using Postgate's medium and subjected to PCR with primers targeting the bacterial 16S rRNA gene. High-throughput amplicon sequencing and bioinformatics tools were used to characterize bacterial communities in PW_N samples. A total of 263,332 valid sequences were analyzed and assigned to 47-79 bacterial OTUs. Alpha-diversity (Table 1) and the Venn diagram (Figure 1) showed some differences between the investigated samples. In general, the bacterial community in sample PW_N4 was more diverse. OTUs common among all samples were limited (Figure 1), which can be explained by the difference in the chemical properties of the water samples. The microbial compositions of PW_N5 demonstrated greater individuality among all studied samples (38% of unique OTUs were characteristic of this sample). The taxonomic composition of the enrichment cultures was determined by amplicon sequencing of bacterial 16S rRNA genes. The results showed that the bacteria in the produced water samples could be divided into several phyla, including the important phylum _Firmicutes_ for samples PW_N1, PW_N2, and PW_N5. _Proteobacteria_ was the dominant phylum for samples PW_N3 and PW_N4, while members of _Firmicutes_, _Proteobacteria_, _Actinobacteria_, _Bacteroidetes_, and _Synergistetes_ were detected in enrichments of PW_N4 and PW_N5 (Figure 2A). Figure 2B illustrates the class-level classification of bacterial communi \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **OTUs** & **Chao1** & **Shannon** & **Simpson** & **PD Whole Tree** & **Fisher’s Alpha** \\ \hline PW\_N1 & 47 & 54 & 2.56 & 0.73 & 3.55 & 5.34 \\ PW\_N2 & 47 & 47 & 2.09 & 0.64 & 3.90 & 4.95 \\ PW\_N3 & 49 & 51 & 1.81 & 0.39 & 4.35 & 5.28 \\ PW\_N4 & 79 & 80 & 3.68 & 0.87 & 7.32 & 9.11 \\ PW\_N5 & 77 & 78 & 2.41 & 0.62 & 7.20 & 8.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Alpha diversity of enriched bacterial communities obtained from produced waters from oil wells near Nurlat. ties. _Clostridia_ was the important class of the phylum _Firmicutes_ in samples PW_NI, PW_N2, and PW_N5 (with relative abundances of 99%, 99%, and 64%, respectively), whereas it was not major in samples PW_N3 (6%) and PW_N4 (10%).