Abstract
BACKGROUND: Hydraulically fractured shales offer a window into the deep biosphere, where hydraulic fracturing creates new microbial ecosystems kilometers beneath the surface of the Earth. Studying the microbial communities from flowback fluids that are assumed to inhabit these environments provides insights into their ecophysiology, and in particular their ability to survive in these extreme environments as well as their influence on site operation e.g. via problematic biofouling processes and/or biocorrosion. Over the past decade, research on fractured shale microbiology has focused on wells in North America, with a few additional reported studies conducted in China. To extend the knowledge in this area, we characterized the geochemistry and microbial ecology of two exploratory shale gas wells in the Bowland Shale, UK. We then employed a meta-analysis approach to compare geochemical and 16S rRNA gene sequencing data from our study site with previously published research from geographically distinct formations spanning China, Canada and the USA. RESULTS: Our findings revealed that fluids recovered from exploratory wells in the Bowland are characterized by moderate salinity and high microbial diversity. The microbial community was dominated by lineages known to degrade hydrocarbons, including members of Shewanellaceae, Marinobacteraceae, Halomonadaceae and Pseudomonadaceae. Moreover, UK fractured shale communities lacked the usually dominant Halanaerobium lineages. From our meta-analysis, we infer that chloride concentrations play a dominant role in controlling microbial community composition. Spatio-temporal trends were also apparent, with different shale formations giving rise to communities of distinct diversity and composition. CONCLUSIONS: These findings highlight an unexpected level of compositional heterogeneity across fractured shale formations, which is not only relevant to inform management practices but also provides insight into the ability of diverse microbial consortia to tolerate the extreme conditions characteristic of the engineered deep subsurface.