Djuna Marie Gulliver ^1* Elliott Barnhart ² Hannah Schweitzer ¹ Heidi Jean Smith ³ David Midgley ⁴

• ¹ National Energy Technology Laboratory (DOE), Pittsburgh, United States
• ² United States Geological Survey, Helena, United States
• ³ Montana State University, Bozeman, Montana, United States
• ⁴ Commonwealth Scientific and Industrial Research Organisation (CSIRO), Canberra, Australian Capital Territory, Australia

As important as this work is to drive innovation in the energy sector, this work also deepens our understanding of subsurface microbial dynamics and these unique, extreme ecosystems. Earth's subsurface environments are one of the largest, yet least studied biomes, due in part to the inability to access relevant biological samples from these uncharted depths. With an industrial motive, however, wells are drilled, and subsurface material is collected with the potential for research collaboration. With advancements in DNA/RNA sequencing and innovative sampling methods, scientists are now able to explore the microbial communities in the hard-to-access geomicrobiological systems. Subsurface microbial communities have evolved adaptations for survival in nutrient-limited, high-pressure, and lowoxygen conditions, offering insights into ecology, evolution, and metabolic pathways in deep biospheres. Recent studies have broadened our knowledge of microbial diversity and function in geological environments, informing fields from astrobiology to environmental science. As we uncover the metabolic networks of these subsurface communities, we find new understanding in microbial genetics and taxonomy that contributes fresh data and novel diversity to our growing catalogue of microbial life.The aim of this Research Topic was to shed light on microbial ecology and physiology in subsurface energy environments, driving new discoveries in biotechnology and insight in microbial interactions of the deep biosphere. The articles presented here did not disappoint in revealing novel findings of the biological subsurface.Two of the manuscripts found surprising microbial impacts in traditional hydrocarbon reservoirs. Stemple et al. identified a newly documented Clostridia species that was highly abundant in the severe conditions of a hydraulically fractured shale reservoir and demonstrated the functional potential of this selected microorganism to drive well souring, biocorrosion, and biofouling. Nai et al. investigated the microbiome of Qinshui Basin coal seams, indicating the unexpected contribution of biogenic processes for methane production on high-rank coals.Two additional manuscripts delve into the unknown effects of microorganisms on subsurface gas storage through investigation of under-characterized subsurface biospheres. Jia et al. identified the importance of rare methanogenic Archaea from the CO2-rich Eger Rift, offering new insights for underground carbon storage. In the pioneering study by Bellini et al., the microbial activity from inoculum of four Italian natural gas reservoirs elucidated much needed insight in the effect native microbial communities may have during underground hydrogen storage.The last two manuscripts touch on the opportunities to use our knowledge in subsurface microbiomes to drive innovative approaches to methane recovery. Platt et al. focuses on testing a promising low carbon renewable coalbed methane enhancement technique using algal amendment across a diverse range of coal ranks. Harirforoush et al. simulated methane gas leakage in Persian Gulf sediments to identify microbial communities consuming methane, enhancing gas exploration. This Research Topic aims to merge the satisfying application of microbiology research to our engineered energy environments with the excitement of novel discovery in the subsurface biosphere. Overall, the findings contribute valuable knowledge for biotechnological applications and gas exploration, as well as new understanding of the capability and contribution of microorganisms in the subsurface. As the subsurface remains the largest and least studied biome, this field of research aims to advance an expected growing awareness of the power microbial communities have within these under-characterized environments.