Unveiling active microbial processes in Earth's deepest seawater.
Microorganisms dominate life in the hadal zone, yet extreme sampling difficulty and low biomass have precluded characterization of their in situ activities. Here, we analyze microbiome samples collected from hadal seawaters via in situ filtration during 12 human-occupied vehicle dives. DNA-protein co-extraction and metagenome-guided metaproteomic analysis identify 135,073 non-redundant active proteins, with over 95% being hadal-specific. Metaproteomic quantification distinguishes highly active and less active taxa that differ in biogeographic origins and genomic traits. Hadal microorganisms operate a metabolic regime fundamentally distinct from the upper ocean, preferentially utilizing refractory organic matter (aromatics, halogenated compounds, and D-amino acids) and expanded electron acceptors (thiosulfate and heavy metals), collectively shaping hadal element cycling. Active viruses extend beyond "Piggyback-the-Winner" dynamics, enhancing host adaptation through auxiliary metabolic genes. These findings provide proteome-level evidence of hadal microbial activities and reveal biogeochemical cycling distinct from that of the upper ocean, highlighting the underappreciated significance of hadal microbiomes within global ocean ecosystems.