Abiotic Sulfurization of Dissolved Organic Matter within the Water Column of the Black Sea

Ocean deoxygenation due to anthropogenic global warming leads to an expansion of oxygen minimum zones (OMZs), with implications for ocean productivity and carbon cycling. The Black Sea is a model system in this context because it is the largest anoxic basin in the world, with sulfidic waters below 150 m depth and dissolved organic matter (DOM) concentration ca. 2.5 times higher than in the open ocean. So far, there is not a consensus about the reasons behind DOM accumulation under O₂-limited conditions. In marine sediments, reduced sulfur species (e.g. H₂S) abiotically react with natural organic matter, contributing to carbon preservation. In this study, we hypothesize that Black Sea DOM is abiotically sulfurized within the water column, contributing to DOM accumulation in anoxic waters. We combined elemental analyses with molecular characterization of DOM using ultra-high resolution mass spectrometry (Fourier transform ion-cyclotron resonance mass spectrometry, FT-ICR-MS). Our results showed that terrigenous DOM represented ca. 20% of Black Sea DOM, being the remaining fraction autochthonous material or derived from the Mediterranean Sea. Quantitative sulfur-per-carbon ratios were more than double below the redoxcline (1.3%) than above (0.6%). The exclusive Black Sea DOM was highly enriched in compounds containing sulfur (43% of all molecular formulae contained sulfur). We established a novel molecular index for the extend of abiotic sulfurization in DOM (“AbiS-compounds”). This index strongly correlated with H₂S concentrations in the Black Sea and other sulfidic environments. Molecular dissimilarity analysis of AbiS-compounds indicated that dissolved organic sulfur was formed in the water column and not derived from sediments. In conclusion, our study provides novel evidence for abiotic sulfurization of DOM in sulfidic waters, emphasizing the potential importance of this process in DOM accumulation in OMZs.