THE DEATH VALLEY REGIONAL FLOW SYSTEM: AN ANALOG FOR SUBSURFACE ROCK-HOSTED ECOSYSTEMS OF ROCKY PLANETS

Thought to contain half of Earth’s microbial biomass, the deep subsurface is the planet’s largest and least understood biome. The Death Valley Regional Flow System (DVRFS) is a vast interconnected fractured-rock aquifer encompassing ~ 100,000 km², beneath portions of Southern and Central NV and Eastern CA, USA. This system can be regarded as a subterranean watershed, where hundreds of springs and boreholes comprise a heretofore-unrecognized regional-scale laboratory for deep life. DVRFS hydrogeologic regimes represent much of the planet’s habitable crust (e.g. volcanic-to-sedimentary) and supports diverse microbial ecosystems that reflect biogeochemical conditions and flow paths identified by regional groundwater models. Chemical, isotopic, and genomic analyses reveal that microbial diversity corresponds to physicochemical drivers; with aerobes dominating oxygenated recharge zones (e.g. Nitrospiraea and Thaumarcheaota), and known deep biosphere-associated lineages (e.g. Hadesarchaea, and Methanothermobacter spp.) occupying hot, anoxic waters of the discharge zone. Unclassified lineages are prominent in some locations, representing up to 40% of the total prokaryotic community. One discharge zone well, intersecting hot (57°C) anoxic fluids at 750 m depth near Death Valley, has been the focus of a whole-life survey targeting viruses to multicellular Eukaryotes. This study, utilizing single cell genomics, metagenomics, metatranscriptomics, and stable isotope probing is revealing an ultra-low-biomass community (~1,000 cells per mL), remarkably similar in composition and inferred function to others previously identified from up to 5 km below land surface in South Africa. Comparative genomics of ‘Candidatus Desulforudis audaxviator’ cells from this site showed nearly identical genomes to individuals from deep wells in Siberia and African gold mines, potentially indicating evolutionary stasis on the order of 100s of millions of years. The existence of this cryptic biome, operating indifferent to modern photosphere inputs, suggests the exciting possibility that rock-hosted biomes may operate within rocky planetary bodies throughout the solar system.