Novel insights into the molecular structure of non-volatile marine dissolved organic sulfur

• Non-volatile marine dissolved organic sulfur (DOS) was molecularly characterized.
• DOS does not contain reduced (e.g., thiols) or hydrolyzable (e.g., thioesters) groups.
• DOS contains sulfonic acids, and possibly thiophenes.
• DOS is highly resistant against oxidation and hydrolyses.
• The molecular structure of DOS seems universal for the oxic water column.

Abiotic sulfurization likely contributes to the preservation of organic matter in fossil deposits. It is unknown whether this mechanism also stabilizes dissolved organic matter (DOM) in the oceanic water column. Knowledge of the structure of the sulfur-containing compounds in DOM could help to understand the observed stability of at least a fraction of refractory DOM in the ocean. Structural analysis of dissolved organic sulfur (DOS) is complicated by it's great molecular diversity and the low concentration of each compound. Two contrasting marine DOM samples, one deep-sea sample from the North Pacific Ocean and one surface sample from the marginal North Sea (Germany), were examined in this study. Selective alteration experiments targeting different sulfur-containing functional groups were applied prior to Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). All treatments were also carried out with corresponding model compounds, in order to test for the applicability of the chosen reactions and for matrix effects, and to confirm the types of changes caused by the treatments. Alteration experiments were composed of harsh hydrolysis, selective derivatization of thiols, oxidation, and deoxygenation. None of these treatments induced detectable changes to DOS, indicating the absence of thioesters, sulfonic acid esters, alkylsulfates, thiols, non-aromatic thioethers, and sulfoxides in solid-phase-extractable DOM (SPE-DOM). Collision-induced fragmentation of isolated nominal masses in the FT-ICR-MS confirmed these findings and revealed the presence of sulfonic acids in both samples. Additionally, the presence of thiophenes and possibly also sulfones is likely, while the presence of any other form of sulfur in SPE-DOM can be ruled out based on our results. In conclusion, only unreactive (fully oxidized and hydrolyzed) sulfur compounds resist in the water column as part of refractory DOM. There was no detectable difference in molecular structure between DOS of the marginal North Sea and the deep North Pacific, indicating that the chemical inertness and the presence of sulfonic acids are global features of DOS residing in the oxic water column.