Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain^1,2,3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle^4,5. Here we use complementary multiplicity-edited ¹³C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds^6,7,8,9,10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OC_qC₃ units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.

溶解有机物 (DOM) 是最复杂、动态和丰富的有机碳来源之一，但其化学反应性仍然不确定^1,2,3。对 DOM 结构特征的深入了解有助于了解其在全球碳循环中的合成、周转和加工^4,5。在这里，我们使用互补多重编辑的¹³ C 核磁共振 (NMR) 谱来量化组装来自亚马逊四条主要河流和两个中等规模的瑞典北方湖泊的 DOM 碳骨架的关键子结构。我们发现一种反应机制，氧化脱芳构化（ODA），广泛用于有机合成化学中以创建天然产物支架^6,7,8,9,10，可能是 DOM 加工过程中产生结构多样性的关键驱动因素，富含合适的多酚前体分子。我们的数据表明，与一个氧原子和三个碳原子（OC _q C ₃单位）结合的四面体季碳含量很高。这些单元在常见生物分子中很少见，但可以通过木质素衍生和单宁衍生多酚的 ODA 轻松生产。 (多)酚通过 ODA 互变异构产生非平面环己二酮，其可进行直接和平行的环加成反应。这种组合导致 DOM 化合物的结构多样性从 DOM 加工的早期阶段开始激增，含氧脂肪族结构也随之增加。总的来说，我们认为 ODA 是加速 DOM 分子加工复杂性、产生新的含氧脂肪族分子的关键反应机制，并且它可能在自然界中普遍存在。