Geochemistry shapes microbial diversity and selected functional traits in flowback and produced waters from hydraulically fractured formations.

Microbial communities inhabiting hydraulically fractured subsurface waters are increasingly recognized as important components of unconventional oil and gas systems because they can influence water quality, infrastructure integrity, and biogeochemical processes during flowback and production. However, a quantitative cross-basin understanding of their taxonomic diversity, ecological organization, and potential functional variation remains limited. In this study, we analyzed 16S rRNA gene amplicons, metagenomes, and geochemical data from flowback and produced water from the Sichuan Basin, China, and conducted a quantitative comparison to data previously reported from the same basin and hydraulic fracturing regions in North America. Our findings revealed strong co-occurrence patterns among fermentative, sulfidogenic, and methanogenic micro-organisms, which emerged as core members of microbial communities across all fractured subsurface environments. Notably, microbial diversity and selected metabolic traits differed across basins in the low-salinity systems of China, whereas high-salinity basins in North America exhibited reduced diversity and more constrained metabolic capabilities. These differences are consistent with salinity acting as an important ecological filter across the analyzed basins. Our results indicate that basin-specific geochemical context, particularly salinity, is closely associated with cross-basin differences in microbial diversity, community composition, and selected metabolic traits in fractured subsurface waters. These findings support the value of integrating geological, geochemical, and microbiological information when interpreting microbial risks and water-management strategies in hydraulic fracturing systems.