## 3 Results and Discussion
### Function of Microbial Communities
Members of the classes _Synergistia_, _Deltaroteobacteria_, _Gammaproteobacteria_, and _Desulfovibronia_, as well as many others, were also observed in the Uzen high-temperature oilfield in Kazakhstan [45]. When analyzing produced water obtained from onshore oilfields in Brazil, representatives of _Enterobacteriales_, _Desulfovibrionales_, and _Pseudomonadadales_ were noted [25]. Bacteria of the obligately anaerobic genus _Desulfotomaculum_ (observed at high levels in samples PW_N1 and PW_N2), as sulfate-reducing bacteria, can grow with sulfate as electron acceptor or ferment in combination with methanogens if sulfate is not available [60]. They are considered important bacterial agents of hydrocarbon degradation, including under relatively high salinity conditions [27] and during steel corrosion [61; 62]. Members of the bacterial genera _Enterobacter_ and _Acinetobacter_ made up the microbial community in sample PW_N3. Previous research work confirmed that _Enterobacteriaceae_ were involved in the biocorrosion of metallic materials under marine conditions [63]. Metal-reducing bacteria of the genus _Shewanella_ are reported to be involved in biocorrosion [64; 65; 66] and were detectedonly in sample PW_N3. On the other hand, the facultative anaerobic bacterium _Shewanella xianenesis_ CQ-Y1, isolated from the wastewater of the Changqing oilfield in China, was proposed by researchers for application in the field of biodegradation and bioremediation. Studies show that _Shewanella_ isolates from different habitats with different physiological and biochemical characteristics, including resistance to high temperatures and high salinity, are relevant for biotechnology [67; 68]. _Desulfovibrio_, _Defluiosulfovibrio_, _Oruithinamicobium_, _Petrimonas_, _Fusibacter_, and _Thermovira_ were important bacterial genera in PW_N4, whereas members of the bacterial genus _Acetobacterium_ and unknown _Microcaceae_ were the major representatives of the microbial community in sample PW_N5. In addition to the already mentioned genus _Desulfotomaculum_, our samples contained strictly anaerobic sulfate-reducing bacteria of the genus _Desulfovibrio_[69] and bacteria of the genus _Dethiosulfovibrio_, capable of reducing thiosulfate and elemental sulfur to hydrogen sulfide [70; 71]. They are described as important microbial agents associated with steel corrosion [72], as well as important representatives of sulfide-rich habitats [73]. [PERSON] et al. [74] investigated the composition of the produced water and enrichment cultures of oil pipelines to study the issue of MIC. Analysis of the produced water demonstrated a high phylogenetic diversity of bacteria and archaea, while enrichment cultures were dominated by bacteria of the dominant genus _Desulfovibrio_ as associated with MIC. Interestingly, synergistic interactions between the mentioned genera _Desulfovibrio_ and acid-producing _Petrimonas_ were reported in a study investigating sulfate-reduction coupling polycyclic aromatic hydrocarbon degradation as a remediation process for contaminated sediments [75]. _Petrimonas_ members as strictly anaerobic bacteria with a fermentative type of metabolism were also detected in anaerobic digesters [57; 58], and their ability to produce organic acids could explain their coexistence with representatives of sulfate-reducing bacteria in some investigated samples. A mesophilic, non-spore-forming, anaerobic, fermentative _Petrimonas sulfuriphila_ capable of using elemental sulfur and nitrate as electron acceptors was first described by [PERSON] et al. [76] as a novel organism isolated from a producing well of a biodegradation oil reservoir in Canada. Bacteria of the genus _Fusibacter_ belong to the class _Clostridia_ and are anaerobic bacteria that can perform thiosulfate reduction. _Fusibacter paucioranus_ was originally isolated from reservoir water from an oil-producing well in the Congo [77].