Organic matter transformation in the upper ocean strongly influences how much carbon is recycled in the water column and how much can be transported to depth. However, the microbial processes linking dissolved organic matter (DOM) composition with carbon turnover remain insufficiently resolved, especially across vertically stratified water columns. This study focuses on paired DOM and microbial samples from three South China Sea stations and collected at six depths. The central question is whether DOM molecular composition and microbial communities show the same coupling pattern across depth, or whether each layer has a distinct DOM–microbe association mode.
DOM molecular composition was characterized using FT-ICR MS, while microbial community composition and functional potential were analyzed using shotgun metagenomic sequencing. According to the observed vertical gradients in DOM molecular composition and microbial community structure, these samples were grouped into three functional layers: surface waters, 50–100 m; mid-depth waters, 300–500 m; and deep waters, 800–1000 m.
Preliminary results show clear vertical differentiation in both DOM composition and microbial communities. Microbial communities shift from surface-associated phototrophic and labile-DOM-responsive groups to mid-depth and deep-water taxa related to nitrogen cycling, complex organic matter utilization, and low-resource adaptation. DOM composition does not change monotonically with depth. Protein-like and amino-sugar-like formulas are relatively enriched at 300–500 m, lipid-like formulas decrease from the surface to the mid-depth layer, and lignin-like or RDOM-like formulas become more important in deeper waters. These patterns support the working hypothesis that the 300–500 m layer is not simply a transition zone, but a key depth interval for DOM reworking.
To further test this hypothesis, depth-specific DOM–microbe association networks are being used to identify which DOM molecular classes and microbial groups co-vary within each layer. Functional genes related to carbon degradation, lipid metabolism, protein and amino acid processing, carbohydrate-active enzymes, and nitrogen cycling will then be used to evaluate whether these associations are consistent with plausible microbial metabolic capacities.
This work is expected to identify layer-specific DOM–microbe association modes in the South China Sea, and address how microbial communities regulate the transformation of organic carbon in the upper 1000 m water column. These findings advance our mechanistic understanding of the microbial carbon pump and provide testable targets for global biogeochemical models, significantly enhancing our predictive capability regarding marine organic carbon migration and storage.