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Summary: Around the world, several dozen deep sedimentary aquifers are being used for storage of natural gas. Ad hoc studies of the microbial ecology of some of them have suggested that sulfate reducing and methanogenic microorganisms play a key role in how these aquifers' communities function. Here, we investigate the influence of gas storage on these two metabolic groups by using high‐throughput sequencing and show the importance of sulfate‐reducing Desulfotomaculum and a new monophyletic methanogenic group. Aquifer microbial diversity was significantly related to the geological level. The distance to the stored natural gas affects the ratio of sulfate‐reducing Firmicutes to deltaproteobacteria. In only one aquifer, the methanogenic archaea dominate the sulfate‐reducers. This aquifer was used to store town gas (containing at least 50% H2) around 50 years ago. The observed decrease of sulfates in this aquifer could be related to stimulation of subsurface sulfate‐reducers. These results suggest that the composition of the microbial communities is impacted by decades old transient gas storage activity. The tremendous stability of these gas‐impacted deep subsurface microbial ecosystems suggests that in situ biotic methanation projects in geological reservoirs may be sustainable over time. [ABSTRACT FROM AUTHOR]

Summary: Using the 16S rRNA and mcrA genes, we investigated the composition, abundance and activity of sediment archaeal communities within 18 high‐mountain lakes under contrasted metal levels from different origins (bedrock erosion, past‐mining activities and atmospheric depositions). Bathyarchaeota, Euryarchaeota and Woesearchaeota were the major phyla found at the meta‐community scale, representing 48%, 18.3% and 15.2% of the archaeal community respectively. Metals were equally important as physicochemical variables in explaining the assemblage of archaeal communities and their abundance. Methanogenesis appeared as a process of central importance in the carbon cycle within sediments of alpine lakes as indicated by the absolute abundance of methanogen 16S rRNA and mcrA gene transcripts (105 to 109 copies g−1). We showed that methanogen abundance and activity were significantly reduced with increasing concentrations of Pb and Cd, two indicators of airborne metal contaminations. Considering the ecological importance of methanogenesis in sediment habitats, these metal contaminations may have system wide implications even in remote area such as alpine lakes. Overall, this work was pioneer in integrating the effect of long‐range atmospheric depositions on archaeal communities and indicated that metal contamination might significantly compromise the contribution of Archaea to the carbon cycling of the mountain lake sediments. [ABSTRACT FROM AUTHOR]

Summary: The bathypelagic ocean is one of the largest ecosystems on Earth and sustains half of the ocean's microbial activity. This microbial activity strongly relies on surface‐derived particles, but there is growing evidence that the carbon released through solubilization of these particles may not be sufficient to meet the energy demands of deep ocean prokaryotes. To explore how bathypelagic prokaryotes respond to the absence of external inputs of carbon, we followed the long‐term (1 year) dynamics of an enclosed community. Despite the lack of external energy supply, we observed a continuous succession of active prokaryotic phylotypes, which was driven by recruitment of taxa from the seed bank (i.e., initially rare operational taxonomic units [OTUs]). A single OTU belonging to Marine Group I of Thaumarchaeota, which was originally rare, dominated the microbial community for ∼ 4 months and played a fundamental role in this succession likely by introducing new organic carbon through chemolithoautotrophy. This carbon presumably produced a priming effect, because after the decline of Thaumarchaeota, the diversity and metabolic potential of the community increased back to the levels present at the start of the experiment. Our study demonstrates the profound versatility of deep microbial communities when facing organic carbon deprivation. [ABSTRACT FROM AUTHOR]