Dominant and Active Methanogens in the Production Waters From a High-Temperature Petroleum Reservoir by DNA- and RNA-Based Analysis

Methane metabolism in deep subsurface petroleum reservoirs is of paramount interest in the global biogeochemical cycle of carbon. Methanogenesis in such habitats is driven mostly by methanogens in syntrophic association with bacteria. In the present study, methanogenic communities in production water samples from six wells of a high-temperature petroleum reservoir were analyzed based on both genomic 16S rDNA and metabolically active 16S rRNA. The PCR-amplified <i>mcrA</i> gene analysis showed that <i>Methanosaeta</i> (26%) and <i>Methanomassiliicoccus</i> (56%) were separately the dominant members in sample W2_71 and W9_18 at the DNA level. In comparison, the RNA-based analysis showed that <i>Methanosaeta</i> (63 ∼ 83%) followed by <i>Methanolinea</i> (16 ∼ 23%) were the most active methanogens, which were different from the communities of genomic DNA. While several lines of studies indicated that CO₂-reducing methanogens of the genus <i>Methanothermobacter</i> was the most frequently detected phylotype in deep-subsurface petroleum reservoirs. Redundancy analysis (RDA) showed the possible correlation between active methanogens (<i>Methanosaeta</i>) and environmental factors (CO₃^2-). The datasets indicated the importance of investigating methanogenic community by integrating DNA- and RNA-based approaches. These results provide new insight into active methanogens in high-temperature petroleum reservoirs, and promote a comprehensive understanding of methanogens as well as their potential methanogenic pathways in such environments.