## 3 Results and Discussion
### Taxonomic Distribution of Bacterial Communities from Oil Wells near Nurlat
Bacteria of the class _Gammaproteobacteria_ (phylum _Proteobacteria_) were enriched in sample PW_N3 (78%) and were also found in samples PW_N4 and PW_N5, but not with high relative abundance values. Representatives of the _Deltaproteobacteria_ were detected in samples PW_N3, PW_N4, and PW_N5. _Actinobacteria_ (12-20%), _Bacteroidia_ (4-13%), and _Synergistia_ (0.6-21%) were detected in samples PW_N4 and PW_N5. The classes _Anaerolineae_, _WS6 (Dojkabacteria)_, _Spirochaeta_, and _Mollicutes_ were noted as minor groups. Members of _Clostridia_ were detected in all samples, with the exception of sample PW_N3, in which the order _Enterobacterialies_ acted as the major group. _Microococales, Bacteroidetes, Desulfoovibrionales, Pseudomonas,_ and _Synergistales_ were common orders in samples PW_N4 and PW_N5. Thus, samples PW_N1, PW_N2, and PW_N3 were poor, and PW_N4 and PW_N5 were richer in bacterial composition. On the family level, the bacterial community in sample PW_N1 was represented by _Peptococcaceae, Lachospiraceae, Clostrtidae 1_, and _Peptococcaceae_. _Caceae_ were major in sample PW_N2, while members of the families _Enterobacteriaceae_ and _Moraxellaceae_ were major in sample PW_N3. The bacterial community detected in sample PW_N4 was more diverse. The main bacterial families were _Desulfoovibrionaceae, Synergistaceae, Intrasporangiaceae, Dysgonomonadaceae_, and _Clostridiales_Family XII_. In sample PW_N5, the main bacterial families were _Eubacteriaceae_ and _Microococcae_. On the genus level, differences between the investigated bacterial communities were very clear. Figure 3 presents the classification of bacterial communities on the genus level. In sample PW_N1, bacteria affiliated with the genus _Desulfoomaculum_ were enriched (46%), followed by bacteria of _Clostridium sensu stricto 3_ (18%), unknown _Lachospiraceae_ (17%), and _Terrisporobacter_ (12%). _Clostridium sensu stricto 3_ and _Desulfoomaculum_ genera were also typical for sample PW_N2, accounting for 53% and 26% of the total sequences, respectively. _Enterobacter_ (60%), _Acinetobacter_ (13%), _Desulfoovibrio_ (8%), _Dethiosulfovibrio_  
Figure 1: The Venn diagram showing the number of unique and shared OTUs of five produced water (PW) samples (1–5) from wells near Nurlat. Overlapped areas depict common OTUs between water samples. (7%), _Desulfotomaculum_ (6%), and _Shewanella_ (3%) were mainly observed in sample PW_N3. For sample PW_N4, _Desulfovibrio_ (27%), _Dethiosulfovibrio_ (15%), _Ornithininicrobium_ (12%), _Petrimonas_ (9%), _Fusibacter_ (8%), and _Thermoviga_ (6%) were important. For sample PW_N5, _Acetobacterium_ (59%), unclassified _Micrococcaceae_ (20%), and _Fusibacter_ (5%) were specific. It is worth noting that there were many 16S rRNA gene sequences that could not be assigned to any known bacterial genera. Figure 2: Relative abundances of bacterial phyla (**A**) and classes (**B**) detected in produced water (PW) samples (1–5) from wells near Nurlat. Despite the use of a selective medium for the enrichment of sulfate-reducing bacteria, representatives of other physiological groups of microorganisms were also detected.