## 3 Results and Discussion
### Function of Microbial Communities  
Reuse and purification of produced water, including reinjecting it back to maintain reservoir pressure and increase crude oil production, are strategies to improve the sustainability of the oil industry. Therefore, great attention must be paid to the quality of  
Figure 9: Relative abundance of bacterial genera detected in produced water (PW) samples (1–2) from wells near Leningorsk. Genera with abundances higher than 0.5% are presented. injected water [53]. Further in-depth research will contribute both to the development of corrosion mitigation technologies and to improving the stability of the oil industry through, for example, the development of MEOR technologies. Some studies have reported relationships between the microbial composition/presence of functional genes in individual water bodies and physical properties (e.g., temperature as a major factor) and chemical properties (e.g., salinity concentration), as well as the availability of essential nutrients [12; 27; 43]. It should be noted that water extracted with crude oil does not reflect the true microbial composition of the oilfield ecosystems studied, as some microbes develop by attachment to the rock matrix or as biofilms at the oil-water interface (as an adaptation of microbes against external environmental factors in oil reservoirs). Thus, in this work, we studied the planktonic microbial communities of an oil-water mixture. Based on the results of the work, individual taxa were identified as important microorganisms with rich functional potential, which opens new opportunities for their implementation in the industrial application of MEOR and bioremediation of oil-contaminated sites [43], as well as biocorrosion monitoring. Despite the impossibility of establishing the origin of microorganisms found in the studied oil reservoirs, we note their excellent adaptation to the physical and chemical conditions of their habitat, which is interesting both from the point of view of their possible indigenous nature and from the point of view of their occurrence as a result of anthropogenic activity. It should be added that despite the use of sterile bottles and sterile instruments, we do not exclude the possibility that a number of bacteria not associated with the oil microbiota could have come from the environment during the sampling stage. The results obtained in this work are consistent with the results of studies in which microorganisms in the oil and water phases of oil production mixtures around the world were analyzed using culture-dependent and culture-independent methods [12; 54; 55]. Representatives of _Proteobacteria_ are considered ubiquitous in oil reservoirs around the world in all temperature ranges [12]. However, they were not widely distributed in all the produced water samples examined. Interestingly, the bacteria within the phyla _Firmicutes_ and _Proteobacteria_ are most actively involved in synthetic interactions with methanogenic archaea in oil reservoirs [12]. Regarding such an important group as sulfate-reducing microorganisms, it is worth noting that this phylogenetically diverse group includes bacteria of the phyla _Proteobacteria_, _Firmicutes_, _Nitrospira_, and _Thermodesulfobacteria_, as well as the archaea belonging to the phyla _Crenarchaeota_ and _Euryarchaeota_[56]. Representatives of the class _Clostridia_ form spores and are the main bacteria in anaerobic systems [57; 58]. Additionally, they are adapted to survive in the harsh conditions of oil reservoirs and are often found in samples from various oil wells [12; 59]. In this regard, the presence of representatives of this class among the bacterial communities of the studied samples was expected. _Gammaproteobacteria_ and _Deltaroteobacteria_ were identified as important microbial groups in samples obtained from the Enermark oilfield in Canada [54] and from the Qinghai oilfield in China [43].