The deep-sea floor, particularly hadal trench systems, represents a critical boundary environment where organic matter undergoes intense transformation under extreme pressure and serves as an important locus of benthic–pelagic connectivity. However, the chemical diversity of sedimentary organic matter and the enzymatic mechanisms governing its transformation in hadal environments remain poorly constrained.
Here we report the widespread occurrence of organically bound halogens in sediments from the Mariana Trench. Bulk measurements show exceptionally high concentrations of organic halogens, corresponding to a halogen-to-carbon stoichiometric ratio of approximately 1:49, indicating that halogenated compounds constitute a substantial fraction of sedimentary organic matter in hadal environments. 
To resolve the functional basis of organohalide transformation, we combined metagenomic analyses, high-pressure incubation experiments, and heterologous expression of trench-derived dehalogenases. Comparative genomic analyses revealed significant enrichment of genes associated with organohalide transformation in hadal sediment microbiomes relative to other marine environments. These functional capacities are widely distributed across 16 microbial phyla and 52 orders, indicating extensive phylogenetic diversification of halogenated compound metabolism in deep-sea sediments.
Importantly, we successfully expressed and purified a representative hadal dehalogenase, and experimentally demonstrated its catalytic activity under low-temperature and high-pressure conditions. The enzyme exhibited broad substrate promiscuity, capable of degrading at least 26 structurally diverse organohalogen compounds, highlighting unexpectedly high metabolic versatility toward halogenated organic matter in extreme environments.
High-pressure microcosm experiments further confirmed rapid transformation of representative organohalide compounds and concurrent activation of dehalogenation pathways under in situ conditions, demonstrating that these enzymatic processes are not only genetically encoded but also functionally expressed within hadal sediment communities.
These findings indicate that sedimentary organohalides represent a reactive and functionally active component of hadal organic carbon pools. Microbially mediated dehalogenation provides a mechanistic link between halogenated organic matter transformation and carbon remineralization, establishing a coupled carbon–halogen cycling pathway in hadal trench sediments. This coupling highlights the role of deep-sea trench microbiomes as active regulators of biogeochemical transformations in the deepest ocean.