Abstract
The deep biosphere encompasses life beneath the Earth's surface and constitutes a substantial portion of the planet's microbial biomass. This study analyzed nucleic acid datasets from low-carbon and low-energy deep terrestrial subsurface groundwaters across four continents and revealed four core global populations. These populations exhibited metabolic strategies and adaptations reflecting depth and environmental constraints. Erythrobacter featured heterotrophic metabolism; Thiobacillus demonstrated sulfur oxidation coupled to denitrification along with carbon and nitrogen fixation; Methanobacteriaceae were methanogenic autotrophs using the Wood-Ljungdahl pathway (WL); and Candidatus Desulforudis audaxviator functioned as a sulfate-reducer also encoding the WL pathway. Depth-related adaptations suggested heterotrophic dominance at shallower depths with increasing contributions from autotrophy with depth. Finally, comparative genomics revealed minimal evolutionary changes among these populations, suggesting functional conservation since diverging from their ancestral lineages. These findings underscore a global deep biosphere core community.