Assessing the methanogenic activity of microbial communities enriched from a depleted reservoir.

Using a depleted gas reservoir as a natural reactor is a novel approach for microbial methanation of hydrogen (H2) and carbon dioxide (CO2) into methane (CH4). This approach, known as underground biomethanation reactor (UMR), could enable the simultaneous valorization of geologically sequestered CO2 and the excess renewable energy, stored in the form of H2 in the same formation as the CO2. In this study, we explore the possibility to trigger biomethanation from formation water sample by testing various carbon sources (CO2, trypticase peptone, glucose, and acetate) in batch test with a defined mineral medium. Obtained results show that trypticase peptone supplementation greatly increased methane production and the enrichment of methanogenic archaea, outperforming alternative carbon sources. 16S rRNA amplicon sequencing of the enriched consortia revealed that supplementation of trypticase peptone and a mixture of H2:CO2 (80:20), resulted in the selection of a mixed culture dominated by microorganisms assigned to the Methanothermobacterium, Garciella, and Caminicella genera. Furthermore, KEGG (Kyoto Encyclopedia of Genes and Genomes) and COG (Clusters of Orthologous Genes) predictive functional analyses underline a possible syntrophic relationship, enhancing the conversion of H2 and CO2 into CH4. This work lays the groundwork for biologically exploiting a depleted gas reservoir by implementing the UMR technology.