Clear niche partitioning of nitrite-oxidizing bacteria from the bottom and the slope of Mariana Trench.

BACKGROUND: The hadal zone, characterized by extreme hydrostatic pressure and geographic isolation, hosts microbial communities uniquely adapted to these harsh conditions. While niche partitioning has been observed in other deep-sea environments, its existence within hadal trench ecosystems remains controversial. Focusing on the Mariana Trench, we investigated whether nitrite-oxidizing bacteria (NOB) exhibit depth-stratified ecological specialization between slope (6000-10,000 m) and bottom (> 10,000 m) sediments. By analysing the genomic features and ecological interactions of NOB, we aimed to resolve their functional roles in nitrogen cycling under distinct hadal microniches. RESULTS: We reconstructed 8 high-quality NOB metagenome-assembled genomes (MAGs) from 58 sediment metagenomes, revealing stark niche differentiation between depth zones. Slope-dominant NOB harboured expanded genetic arsenals for antioxidation (e.g. superoxide dismutase) and osmoprotection (compatible solute transporters), Suggesting enhanced adaptive capacity to pressure-adjacent stresses. Metatranscriptomics revealed 1.48 × (nxrA) and 1.28 × (aclA) greater expression of nitrite oxidation and carbon fixation genes in slope communities than in their bottom counterparts. Network analysis identified slope NOB as keystone taxa with elevated among-module connectivity and intramodule linkages, in contrast with bottom NOB, which exhibited localized nitrate-production gene networks. Functional profiling revealed complementary biogeochemical roles: slope NOB primarily consumed nitrite, whereas bottom populations dominated nitrate synthesis. CONCLUSION: Our multiomics analysis revealed depth-dependent niche partitioning among hadal NOB, with transcriptional and network evidence supporting distinct pressure adaptation strategies and biogeochemical functions. The slope-bottom differentiation in stress response systems and nitrogen transformation pathways highlights how micron-scale environmental gradients drive microbial specialization in Earth's deepest ecosystems. These findings establish NOB as critical mediators of hadal biogeochemical cycles and provide a framework for understanding microbial resilience in extreme biospheres. Video Abstract.