The deep sea is home to a vast and largely unexplored microbial biosphere, along with large amounts of complex organic matter (COM). However, the functional capacity of the deep-sea microbiome to metabolize organic matter across diverse regions remains poorly understood. Here, we combine 16S rRNA gene amplicon sequencing, metagenomics, and metatranscriptomics to comprehensively characterize prokaryotic communities across different years (2018 and 2022) and habitats (cold seeps, hydrothermal vents, and seamounts). Our results reveal spatio-temporal community heterogeneity driven by geochemical gradients, alongside a widespread genetic potential for organic matter metabolism. Notably, the PVC (Planctomycetota-Verrucomicrobiota-Chlamydiota) superphylum exhibits extensive polysaccharide degradation capabilities, exemplified by the isolation of Planctomycetota strain WC338 and Lentisphaerota strain WC36 via laminarin enrichment. Growth experiments and transcriptomics confirm their strict laminarin dependence and characterize the underlying catabolic machinery-specifically, the deployment of different glycoside hydrolase (GH) families, which are broadly distributed and prevalent across the PVC superphylum. Furthermore, we demonstrate that laminarin acts as an effective selective substrate for enriching and isolating the deep-sea PVC superphylum bacteria. Collectively, these findings reveal that PVC bacteria-an overlooked group in laminarin degradation-possess specialized adaptations for polysaccharide breakdown and actively participate in laminarin turnover in deep-sea environments.
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