Diversity and Biotechnological Potential of Nitrate-Reducing Bacteria from Heavy-Oil Reservoirs (Russia)

Nitrates do not occur in the formation water of oil reservoirs. A number of oil companies use nitrate injection technology to suppress corrosion of steel equipment and to decrease the concentration of hydrogen sulfide in the oil reservoir. The efficiency of this technology depends on the presence in the oil reservoir of bacteria capable of reducing nitrate to nitrite, the ion inhibiting growth of sulfate-reducing bacteria (SRB); otherwise, nitrate-reducing bacteria adapted to reservoir conditions must be introduced into the reservoir. In this work, we determined the phylogenetic diversity and potential functional activity of bacterial enrichment cultures reducing nitrate in a medium with oil, which were obtained from Tatarstan oil reservoirs. The bacteria accumulating nitrite in the medium were isolated. Growth of enrichment cultures in the medium with oil was accompanied by formation of nitrite and a decrease in the surface and interfacial tension of the medium. High-throughput sequencing of the V3–V4 region of the 16S rRNA gene of nitrate-reducing cultures revealed that Gammaproteobacteria, represented by bacteria of the genus Halomonas in the 5452-NR culture, and Pseudomonas, Stenotrophomonas, and Escherichia–Shigella in the 245D-NR culture, were predominant. Analysis of the obtained 16S rRNA gene libraries using the iVikodak program made it possible to predict the high potential functional diversity of bacteria in the enrichments capable of denitrification with production of nitrite and gaseous nitrogen compounds, and nitrate reduction with formation of nitrite and ammonium, as well as of catabolizing benzoate and other aromatic compounds. Eight strains of nitrate-reducing bacteria of the genera Chromohalobacter, Halomonas, Bacillus, Nocardia, Gordonia, Ensifer, and Pseudomonas, which grew aerobically on oil n-alkanes, were isolated from the enrichments. Enrichment cultures in media with nitrate and oil probably use the aromatic components of oil. Bacteria of the genera Chromohalobacter and Halomonas may be used for introduction into the oil reservoirs with low abundance of nitrate-reducing bacteria to reduce nitrate to nitrite, decrease the concentration of sulfide, and suppress the formation of hydrogen sulfide by sulfate-reducing bacteria in the reservoir.