The binding of organic carbon (OC) to reactive iron (Fe) phases is a critical mechanism enabling long-term carbon preservation, with the stability of such associations exerting profound influences on global carbon cycling. At redox interfaces, decaying biomass (termed microbial necromass) and microbial byproducts, along with transformed organic matter (OM), recombine with minerals to form mineral-organic assemblies (MOA) that mediate OM sequestration. Notably, microbially derived products constitute a significant fraction of preserved OM—up to ~50% in soils, and similarly substantial proportions in marine sediments and black shales—highlighting the importance of microbial necromass-Fe interactions in carbon stabilization. The preservation potential of iron-bound microbial derivatives warrants explicit incorporation into global carbon budget models. However, previous research has predominantly focused on terrestrial systems, with insufficient biological and mechanistic insights into marine sedimentary environments—particularly in hadal trenches (>6,000 m depth)—where necromass cycling remains severely underestimated. This study addresses the destabilizing role of microbial dissimilatory iron reduction (MDIR) in modulating the stability of iron-bound microbial necromass. Utilizing Shewanella sp. MTB7—a hadal zone bacterium isolated from the seawater-sediment interface at 6,000 m depth that couples MDIR activity with siderophore production—we mechanistically demonstrate how iron reduction and siderophore-driven mineral dissolution synergistically liberate iron-bound organic carbon. By establishing a quantitative framework for necromass-mineral interaction dynamics, this work advances the theoretical foundation for iron-mediated carbon sequestration geoengineering. Furthermore, it provides empirical constraints to reconcile centennial-scale carbon mineralization with millennial-scale burial efficiency, enabling precise integration of microbially driven iron-carbon decoupling into Earth system models for global carbon flux quantification. These insights are critical for refining predictive assessments of marine carbon sink resilience under shifting oceanic redox regimes and anthropogenic perturbations.

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