The deep microbial biosphere is a vast ecosystem extending kilometers beneath Earth's surface in terrestrial rocks, aquifers, marine sediments, and oceanic crust. Blocked from sunlight, and often severely limited in organic carbon and oxygen, these environments host microorganisms sustained by energy sources such as hydrogen gas, reduced metals, and sulfur compounds. Over the past few decades, advances in sampling, cultivation-independent methods, and deep-sea drilling have revealed diverse bacterial, archaeal, and eukaryotic life in the subsurface, including still poorly understood lineages with unusual metabolic strategies. Despite extreme energy limitation and often harsh conditions, subsurface microorganisms persist through adaptations and strategies such as metabolic versatility, streamlined genomes, efficient carbon fixation pathways, and slow growth. Collectively, microbial communities exert an important yet poorly constrained influence on Earth's long-term carbon, sulfur, and nitrogen cycles. Although uncertainties remain related to rates of metabolic activity, generation times, and global contributions to biogeochemical cycles, the deep biosphere has emerged as a central component of Earth's biosphere and is growing in relevance as human activities increasingly intersect with the deep subsurface.
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