Universal molecular structures in natural dissolved organic matter

The molecular composition of marine dissolved organic matter (DOM) is similar across the globe, all the more so when analyzed at highest analytical resolution with novel techniques such as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Despite geographical gradients in climate, nutrient regime, microbial community structures, and other factors, a similar suite of molecular formulae are detected in different environments. Even terrigenous DOM, with large contributions of vascular plant debris, shares thousands of molecular formulae with marine DOM. But to this point, it remains uncertain whether this globally common pool of molecular formulae also represents identical molecular structures. In principle, many structural isomers exist for a given molecular formula, and a wide range of structures may be represented by a single molecular formula. The objective of this study was to compare DOM of vastly different origin from land to deep ocean on a molecular formula and structural level. For this purpose, we performed collision-induced fragmentation experiments in an FT-ICR-MS. These experiments yielded detailed fingerprints for single molecular formulae that are highly characteristic for specific structural features. Experiments with model compounds showed that the method is sensitive towards minor structural differences among molecules, e.g. the position of a carboxyl-group on an aromatic ring. Surprisingly, identical DOM molecular formulae showed also indistinguishable structural fingerprints, independent of the origin of the sample. Molecular formulae that were unique to a certain type of environment, however, showed often different fragmentation pattern. Our results indicate that there is a universal core component in DOM that is structurally indistinguishable between aquatic environments even with the most advanced analytical methods. This homogeneity could be the result of a cascade of degradation processes that ultimately leads to the formation of a common refractory background signal in DOM.