Deep terrestrial indigenous microbial community dominated by Candidatus Frackibacter. Deep terrestrial fracture fluid

Characterizing deep subsurface microbial communities informs our understanding of Earth’s biogeochemistry as well as and the potential for subsurface life elsewhere in the solar system. Here we characterized microbial communities within the Kidd Creek Observatory subsurface fracture water system with mean residence times of hundreds of millions to over one billion years. 16S rRNA analysis revealed that biosamplers well isolated from the mine environment were dominated by a putatively anaerobic and halophilic bacterial species from the Halobacteroidaceae family, Candidatus Frackibacter. Contrastingly, biosamplers and biofilms exposed to the mine environment putatively aerobic Sphingomonas taxa. δ13C of phospholipid fatty acids and putative functional predictions derived from 16S rRNA gene profiles, showed the importance of anaerobic chemoheterotrophy by Candidatus Frackibacter, implying the use of carbon derived from ancient carbon-rich layers common in these systems. These results indicate that Candidatus Frackibacter is not unique to hydraulically fracked sedimentary basins but rather may be indigenous, along with other members of the Halobacteroidaceae, to the subsurface and able to survive in a wide range of deep, saline groundwaters hosted in carbon-rich rocks. Deep subsurface environments may be a refuge for life on Earth, with implications for saline waters in the subsurface of other planets and moons.