Microorganisms are known to live in the deep subsurface, kilometers below the photic zone, but the community-wide metabolic networks and trophic structures (the organization of their energy and nutritional hierarchy) remain poorly understood. We show that an active subsurface lithoautotrophic microbial ecosystem (SLiME) under oligotrophic condition exists. Taxonomically and metabolically diverse microorganisms are supported, with sulfur-driven autotrophic denitrifiers predominating in the community. Denitrification is a highly active process in the deep subsurface that evaded recognition in the past. This study highlights the critical role of metabolic cooperation, via syntrophy between subsurface microbial groups, for the survival of the whole community under the oligotrophic conditions that dominate in the subsurface.