Introduction<\/h3>\n\n\n\n
The high worldwide demand for crude oil led to enhanced oil recovery and more effective oil extraction from reservoirs. However, among the tertiary methods of oil recovery enhancement, implementing microorganisms is of great interest. This approach, called microbiological enhanced oil recovery (MEOR), has several advantages, such as environmental safety and an economically low cost1<\/a>,2<\/a><\/sup>. MEOR technologies use both, microorganisms and their metabolites, including biosurfactants, biopolymers, bioacids, solvents, biogas, and enzymes, to produce residual oil3<\/a>,4<\/a><\/sup>. Nevertheless, biosurfactants stand out promisingly among metabolites with the highest prospects for use in MEOR5<\/a><\/sup>.<\/p>\n\n\n\n Microorganisms can produce various biosurfactants that can replace chemical analog compounds. Biosurfactants are amphiphilic compounds with a hydrophilic end and a hydrophobic end, separated at the interface between different phases6<\/a>,7<\/a><\/sup>. In addition, from the perspective of MEOR applications, biosurfactants have many advantages, such as biodegradability, low toxicity, low critical micelle concentration (CMC), and the ability to be obtained from renewable and cheap substrates8<\/a>,9<\/a><\/sup>. Biosurfactants are known for their excellent surface activity, which also includes reducing the surface and interfacial tension between different phases (liquid\u2013air, liquid\u2013liquid, and liquid\u2013solid). The ability to reduce surface and interfacial tension is achieved due to the adsorption of biosurfactants at different phases, which leads to more significant interaction and mixing of dissimilar phases. Consequently, they have an emulsifying activity that can effectively emulsify two immiscible liquids, such as hydrocarbons or other hydrophobic substrates10<\/a><\/sup>. Biosurfactants have a wide range of biotechnological applications in the oil industry. They work better than chemical analogs due to their higher compatibility with the environment, reduced toxicity, and the ability to be produced from renewable sources. Biosurfactants mainly contribute to reducing the interfacial tension between oil-bearing rocks and oil solution, changing the wettability of porous media, emulsifying crude oil, and affecting the oil, making it even more fluid-like11<\/a>,12<\/a><\/sup>.<\/p>\n\n\n\n Microorganisms synthesize various types of biosurfactants, which are classified based on their chemical composition and microbial origin, that includes glycolipids and lipopeptides, lipoproteins, phospholipids and fatty acids, polymeric surfactants, and particulate surfactants13<\/a><\/sup>. Glycolipids and lipopeptides are low-molecular biosurfactants that reduce surface and interfacial tension, while other high-molecular biosurfactants (e.g., lipoproteins, lipopolysaccharides, and amphipathic polysaccharides) are effective in stabilizing emulsions14<\/a>,15<\/a><\/sup>. Pseudomonas<\/em> spp., Bacillus<\/em> spp., Rhodococcus<\/em> spp., Candida<\/em> spp., and Acinetobacter<\/em> spp. are among the well-known bacterial manufacturers of biosurfactants16<\/a>,17<\/a>,18<\/a>,19<\/a>,20<\/a><\/sup>. When applying MEOR associated with the production of metabolic products, it is necessary to consider some factors that significantly affect the results of the process (geological and physical properties of the reservoir, including temperature, pressure, salinity, pH, and composition of reservoir water)2<\/a>,21<\/a><\/sup>.<\/p>\n\n\n\n In the context of MEOR, this study focused on screening and studying bacteria producing biosurfactants from the formation water of Akingen oil reservoirs in Western Kazakhstan (Fig. 1<\/a>). The selected bacteria, aligned with MEOR criteria, were tested for biosurfactant and biopolymer production, and their phylogenetic relationship. Additionally, the study evaluated the presence of biosurfactant and biopolymer genes and their expression under different conditions. The relevance of this research is underscored by challenges posed in oil fields, including high water cut (80\u201390%), significant untapped oil reserves (up to 60\u201370%) in deep reservoirs, high oil viscosity, low reservoir permeability, and complex geological structures in the oil fields in Kazakhstan. Moreover, in Kazakhstan, most of the hydrocarbon deposits have already been discovered and commercially produced. In this regard, the residual oil left behind in these mature hydrocarbon deposits after primary and secondary oil recovery offers an opportunity to implement EOR processes, including the application of microbial-enhanced oil recovery (MEOR) technology.<\/p>\n\n\n
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