Microbial community composition and diversity in production water of a high-temperature offshore oil reservoir assessed by DNA- and RNA-based analyses

Microbial activity is ubiquitous and important in petroleum reservoir systems; however, current knowledge regarding the diversity and functions of microbial communities that are active in offshore petroleum reservoirs remains very limited. In the present work, high-throughput sequencing analysis of the 16S rRNA gene (DNA) and transcripts (RNA) was applied to respectively characterize the genomic and active microbial communities from two production wells in a high-temperature offshore oilfield in China. The results showed that the microbial community compositions in the two wells (X1 and X2) were very different. At the genus level, the dominant members in both genomic and active communities of sample X1 were Bosea, Acinetobacter, Sphingomonas, Candidatus Methanofastidiosum and Methanocalculus. Both genomic and active communities in sample X2 were mainly represented by Pseudomonas, Acinetobacter, Methanothermobacter, Methanosaeta, Candidatus Nitrosopumilus, Candidatus Methanomethylicus, Thermococcus, Candidatus Methanofastidiosum and Methanocalculus. Moreover, using an RNA-based approach, genera with a very low abundance and even undetected genera (based on a DNA approach) were shown to have increased activity. Besides the abundant active members, the comparison between the DNA- and RNA-based datasets also permitted the identification of genera that were relatively more active in the two production water samples. The existence of these active microorganisms suggested they might have potential functions associated with reservoir issues in the offshore oilfield. This study highlights the importance of assessing microbial diversity through a combination of DNA- and RNA-based analyses, and promotes a comprehensive and deep understanding of microbial compositions and their roles in offshore petroleum reservoirs.