Functional and phylogenetic microbial diversity in formation waters of a low-temperature carbonate petroleum reservoir

Functional and phylogenetic microbial diversity in formation waters of carbonate low-temperature bed 302 of the Romashkinskoe oil field was studied using microbiological, molecular and radioisotope techniques. Cultivable sulfate-reducing prokaryotes (103–106 cells ml−1) predominated in formation water and the process of sulfate reduction was paramount. The rate of sulfate reduction varied within the range 2.28–26.60 μg S2− l−1−day−1. The number of fermentative bacteria varied from 102 to 104 cells ml−1, of methanogens – from 10 to 104 cells ml−1. Methanogenesis was a minor process with the rate not exceeding 0.395 μg CH4 l−1 day−1. 16S rRNA gene clone libraries for Bacteria and Archaea were constructed on the basis of DNA extracted from formation water. From the bacterial clone library, 16S rRNA genes of aerobic organotrophs (genera Pseudomonas, Acinetobacter, ‘Brachymonas’ etc.), anaerobic organotrophs (genus Thauera, Bacteroidetes) and sulfate reducers (Desulfoglaeba, Desulfomicrobium, Desulfovibrio and Desulfobacterales) were retrieved. For the first time, the predominance of nucleotide sequences of the bacterium Desulfoglaeba alkanexedens known to grow on n-alkanes was revealed in the clone library from the carbonate oil reservoir. Archaeal sequences in the library of formation water were not attributable to any recognized methanogens. Both the sulfate-reducing bacterium D. alkanexedens and syntrophic assemblages could be involved in biodegradation of crude oil associated with sulfate reduction in the carbonate petroleum reservoir. ► Microorganisms of carbonate petroleum reservoir (Russia) were studied. ► Fermentative and sulfate-reducing bacteria were predominant cultivated groups. ► Sulfate reduction is a major process in produced water samples. ► 16S rRNA genes of Desulfoglaeba dominate in the clone library of water samples. ► Both sulfidogenic assemblages and sulfate-reducer Desulfoglaeba degrades oil in the oil field.