The sea surface microlayer (SML) is the boundary between the atmosphere and the ocean surface and is central to the physical, chemical, and biological exchange processes that connect the atmosphere and oceans. Dissolved organic matter (DOM) in the SML may contain surface-active and bioavailable components that modify interfacial physicochemical properties and potentially influence air–sea gas exchange. As an important component of the DOM, surface-active substances (SAS) can accumulate at the air–water interface, where they may suppress near-surface turbulence and gas-transfer velocity, ultimately affecting air–sea CO₂ fluxes. However, the molecular composition of SML-DOM and its linkage to potential surface-active components remain insufficiently constrained, particularly under contrasting Hydrological conditions. In this study, paired SML and subsurface water (SSW) samples were collected from two representative stations in the Northwestern Pacific in September 2024, including an eddy-edge station and an off-eddy station. Bulk organic matter parameters, optical properties, total dissolved amino acids (TDAA), and ultra-high-resolution mass spectrometry (FT-ICR MS) were integrated to characterize the enrichment behavior, bioavailability, and molecular composition of SML-DOM.

    The results showed that DOC, POC, PN, and fluorescent DOM components were enriched in the SML relative to SSW at both stations, indicating the accumulation of organic matter at the air–sea interface. Nevertheless, the composition of enriched DOM differed markedly between the two hydrological settings. At the eddy-edge station, the SML exhibited pronounced enrichment of highly bioavailable and potentially surface-active DOM molecules, as indicated by elevated TDAA carbon-normalized yields [TDAA (%DOC)], enhanced protein-like fluorescence, and greater abundances of unsaturated aliphatic and peptide-like compounds. In contrast, the SML at the off-eddy station preferentially accumulated more refractory DOM, characterized by lower TDAA(%DOC), higher humification index, increased aromaticity, and higher abundances of condensed aromatics and polyphenols with strong photochemical activity.
    These findings demonstrate that mesoscale physical processes may regulate the selective enrichment of DOM in the SML and reshape the molecular composition of interfacially active organic matter. The enrichment of peptide-like, unsaturated aliphatic, and N/S-containing heteroatomic molecules provides molecular-level evidence for identifying potential precursors of natural surface-active substances. This study highlights the importance of integrating DOM molecular composition, surfactant activity, and gas transfer measurements to better evaluate the role of SML organic matter in modulating air–sea exchange processes.