[Department of Ocean Science] PhD Thesis Defense Seminar: Dissolved Organic Nitrogen Cycling in Marine Environments: From Molecular Fingerprints to Thermodynamic Stability

Abstract: 

Marine dissolved organic nitrogen (DON) is a critical yet enigmatic reservoir in global biogeochemical cycles. Tracing its fate is hindered by its immense molecular complexity and the analytical challenges associated with measuring stable nitrogen isotopes (δ15N). This thesis integrates δ15N, ultra-high-resolution mass spectrometry (FT-ICR MS), machine learning (ML), and thermodynamic calculations to decode DON cycling from coastal margins to the global ocean. First, a dual-proxy approach revealed distinct DON trajectories in Chinese coastal environments. In the Yangtze River Estuary, mixing drove DON addition and the molecular nitrogen transfer from CHON1 to CHON3 (where CHONn denotes N-containing molecular formulas with n nitrogen atoms). Conversely, in the Yellow Sea Cold Water, in-situ DON degradation (with an isotope effect of 2.5‰) produced recalcitrant carboxyl-rich alicyclic molecules (CRAM) via a reverse CHON2 to CHON1 transformation. To deconvolute this molecular complexity, weighted correlation network analysis was applied, clustering the DON pool into four universal functional modules. Projecting these modules globally revealed a striking functional convergence in marine DON cycling driven by salinity and temperature. To overcome δ15N measurement bottlenecks, a support vector regression ML model was developed to predict DON δ15N directly from FT-ICR MS data (mean absolute error = 0.32‰). The model demonstrated that even nitrogen-free formulas (CHO) encode critical coupled C-N processing information essential for predicting δ15N. Finally, the millennial-scale persistence of marine refractory dissolved organic matter (RDOM) and refractory DON (RDON) was addressed through a novel thermodynamic framework. By calculating the Gibbs free energy difference between molecular production and consumption (ΔGpc), results suggest that RDOM/RDON stability is not a consequence of chemical inertness, but reflects a dynamic near-steady state (ΔGpc ≈ 0) governed by microbial thermodynamics. Collectively, this thesis advances marine DON research from descriptive molecular fingerprints to predictive and mechanistic frameworks, providing vital insights into the ocean's long-term carbon and nitrogen sequestration.