Molecular properties of deep-sea dissolved organic matter are predictable by the central limit theorem: evidence from tandem FT-ICR-MS

The reason behind the millennial stability of marine dissolved organic matter (DOM) is subject of controversial discussion. It may be derived by either the occurrence of mainly stable chemical structures or concentrations of individual DOM molecules too low for efficient microbial growth. One of the major challenges in solving this enigma is that, to date, full structural elucidation of DOM remains impossible. Ultrahigh-resolution mass spectrometry can resolve the composition of DOM on a molecular formula level, but the molecular diversity of the isomers behind each formula is unknown. The objective of our study was to fill this gap of knowledge. Molecular fragmentation experiments were performed via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) on single substances and molecular formulae isolated from deep-sea DOM. We estimated carboxyl (COOH) content with help of two novel measures, one derived from molecular formula information alone and one from fragment ion intensities. For the carboxyl content of single substances, they were poor predictors and fragment ion intensities were highly sensitive to structural properties. For natural DOM, on the contrary, both parameters were highly significantly correlated, despite obvious structural differences between the molecular formulae considered in this study. By using a model approach based on the central limit theorem, we were able to show that the observed fragment ion intensities of DOM may be explained by intrinsic averaging. These results are clear experimental evidence that many isomers exist per molecular formula. Model calculations showed that the observable molecular properties of DOM apparently emerged as averages of multiple isomers according to the central limit theorem. Structural differences between isomers that would reduce the accuracy of our measures for carboxyl content lost their effect due to averaging. Our model calculations based on the central limit theorem indicated that there are at least 100,000 different compounds in DOM each present in seawater at picomolar concentrations