Depth constrains carbohydrate turnover in mangrove sediments through microbial assembly and functional loss

Carbohydrate preservation in mangrove sediments underpins blue carbon storage, yet the biological processes shaping its fate remain unresolved. In our study, we employed depth-resolved shotgun metagenomics (0–100 cm) to investigate microbial community assembly and carbohydrate-active enzyme (CAZyme) dynamics in mangrove sediments. We reveal stochastic assembly dominating surface layer communities, transitioning to deterministic processes in deeper sediments, paralleled by pronounced declines in lignin-, peptidoglycan-, and glucan-degrading CAZymes, which elucidate mechanisms for long-term carbon sequestration. To achieve a more comprehensive characterization of CAZyme repertoires, we reconstructed 363 medium- to high-quality metagenome-assembled genomes. By integrating geochemical analyses with gene-coding density assessments, we revealed Anaerolineae as dominant anaerobic specialists sustaining limited carbohydrate metabolism under nitrogen-limited and anoxic deep-sediment conditions. These depth-driven patterns reveal an ecological filter shaping microbial communities and enzymatic functions that sustain blue carbon in mangrove sediments.