Cascade Reservoirs Regulate Molecular Signatures and Biogeochemical Pathways of Dissolved Organic Matters in the Lancang River

Dissolved organic matter (DOM) serves as a critical biogeochemical nexus in river systems, yet its compositional restructuring under reservoirs remains poorly constrained. Through comparative Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis of the Lancang River’s natural and impounded reaches, we identified 9,410 molecular formulas that reveal systematic reservoir-induced transformations in DOM architecture. Notably, 531 compounds were reservoir-specific, exhibiting distinct molecular signatures with reduced double bond equivalence (DBE), elevated H/C ratios, and enrichment in peptide-/lipid-like components, indicative of enhanced autochthonous production. Phytoplankton community analysis revealed the reservoir-specific dominance of Chlorophyta and Cyanophyta, whose metabolic activities promoted the biosynthesis of aliphatic components and unsaturated compounds. Reaction networks uncover intensified anaerobic processing in reservoirs, where demethylation and dehydrogenation pathways generate methane precursors, potentially amplifying CH4 flux. Carbon oxidation state (NOSC) emerges as a key molecular selector, with redox-selected preservation of low-energy CHO compounds (NOSC < 0) forming refractory carbon pools, while bioavailable CHON species demonstrate significant correlations of sediment organic carbon with biological index and peptide-like content. These molecular-level insights suggest that cascade reservoirs simultaneously process organic matter through microbial carbon pumps and sequester carbon via selective molecular preservation, benefiting from optimizing hydropower management in riverine ecosystems.溶解性有机质（Dissolved Organic Matter, DOM）是河流生态系统中关键的生物地球化学纽带，但其在水库作用下的组成重构机制仍缺乏明确约束。本研究通过对澜沧江天然河段与库区河段开展对比傅里叶变换离子回旋共振质谱（Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, FT-ICR MS）分析，共鉴定出9410个分子式，揭示了DOM结构受水库作用产生的系统性转变。值得注意的是，其中531种化合物为库区特有，表现出独特的分子特征：双键等价（Double Bond Equivalence, DBE）降低、氢碳比升高，且肽类/脂类组分富集，这暗示内源生产过程得到增强。浮游植物群落分析显示，库区特有优势类群为绿藻门（Chlorophyta）与蓝藻门（Cyanophyta），其代谢活动促进了脂肪族组分与不饱和化合物的生物合成。反应网络分析揭示了水库内强化的厌氧过程：脱甲基与脱氢通路可生成甲烷前体物，或进一步加剧甲烷（CH4）通量。碳氧化态（NOSC）作为关键的分子筛选因子，经氧化还原筛选保留的低能CHO类化合物（NOSC < 0）形成了难降解碳库；而生物可利用的CHON类物质则与沉积物有机碳、生物指数及肽类组分含量呈显著相关。这些分子层面的研究结果表明，梯级水库（cascade reservoirs）可同时通过微生物碳泵对有机质进行转化，并通过选择性分子留存实现碳固存，这为河流生态系统中的水电管理优化提供了有益参考。