## 3 Results and Discussion
### Taxonomic Distribution of Bacterial Communities from Oil Wells near Leningorsk  
After the immersion test, the diversity of bacterial communities was studied. Total DNA was isolated from enrichments (PW_L1_stimul and PW_L2_stimul) and subjected to PCR with primers targeting the bacterial 16S rRNA gene. A total of 163,591 valid sequences were analyzed and assigned to 108-125 bacterial OTUs. Alpha-diversity analysis (Table 3) and the Venn diagram (Figure 7) showed several differences between the samples investigated. The indices also indicated that bacterial communities associated with samples PW_L1 and PW_L2 were more diverse compared to samples PW_N1-PW_N5. Differences may also be due to the use of different cultural media. Shared OTUs were high in both samples, although each oil well also contained unique bacterial OTUs (Figure 7). The bacterial compositions of PW_L2 demonstrated greater individuality among both analyzed samples (25% of unique OTUs were characteristic of this sample). The taxonomic composition of the enriched bacterial communities in samples PW_L1 and PW_L2 was then investigated. The results demonstrated that the bacteria in the produced water samples could be classified into several phyla (Figure 8A). Thus, sample PW_L1 contained representatives of _Frimicutes_ (23%), _Bacteroidetes_ (22%), _Proteobacteria_ (19%), _Spirochaetes_ (16%), and _Synergistetes_ (11%), as well as members of many other minor phyla. Sample PW_L2 contained representatives of _Synergistetes_ (31%), _Frimicutes_ (24%), _Spirochaetes_ (13%), _Proteobacteria_ (10%), and _Patescibacteria_ (9%) as well as members of many \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\_L1 & 108 & 114 & 4.16 & 0.87 & 7.28 & 12.30 \\ PW\_L2 & 125 & 133 & 4.02 & 0.88 & 8.76 & 14.43 \\ \hline \end{tabular}
\end{table} 
Table 3: Alpha diversity of enriched bacterial communities obtained from produced waters from oil wells near Leningorsk. Figure 7: The Venn diagram showing the number of unique and shared OTUs of two produced water (PW) samples (1–2) from wells near Leningorsk. Overlapped areas depict common OTUs between water samples. other minor phyla. Figure 8B shows the classification of bacterial communities on the class level. Thus, sample PW_L1 was mainly characterized by _Bacteroidia_, _Clostridia_, _Deltaproteobacteria_, _Leptospirae_, and _Synergistia_, while sample PW_L2 was mainly characterized by _Synergistia_, _Clostridia_, _Spirochaeta_, _Deltaproteobacteria_, _Parcubacteria_, and _Coriobacteria_. _Campylobacteria_, _Thermotogae_, _Mollicutes_, _Anaerolineae_, _Bacilli_, and _Actinobacteria_ were noted as minor classes. On the family level, the bacterial community in sample PW_L1 was predominantly represented by _Desulfomicrobiaceae_, _Leptospiraeae_, _Synergistaceae_, _Dysgonomonasaceae_, _Eubacteriaceae_, _Lachnospiraeae_, _Clostridiales_Family XII_, and _Petrotogaceae_, as well as many other families. The bacterial community in sample PW_L2 was mainly represented by _Synergistaceae_, _Sirochaetaaceae_, _mcultured _Moranbacteria_, _Defluvitatelaceae_, _mcultured _Coriobacteria_, _Desulfomicrobiaceae_, _Lachnospiraceae_, _Costiridiaceae_, _Desulfobacteraceae_, as well as many other families. On the genus level (Figure 9), the bacterial community in  
Figure 8: Relative abundance of bacterial phyla (**A**) and classes (**B**) detected in produced water (PW) samples (1–2) from wells near Leniogorsk. Phyla and classes with abundances higher than 0.5% are presented. sample PW_L1 was predominantly represented by uncultured _Leptospiraceae_ (13%), _Fermentomonas_ (10%), _Desulfomicrobium_ (10%), uncultured _Marinilabiliaceae_ (8%), _Thermovigra_ (8%), _Desulfoplanes_ (7%), _Dethiosulfatibacter_ (5%), _Acetobacterium_ (5%), _Fusibacter_ (4%), and uncultured _Lachospiraceae_ (4%), as well as many other minor genera and unknown bacteria.