Influence of oxygen concentration on the elemental and molecular composition of marine dissolved organic matter in anoxic basins

Over the last 50 years, the permanently anoxic regions in the ocean have quadrupled in size due to deoxygenation derived from global warming and climate change. Marine anoxic basins are characterized by strong vertical redox variability. In the deep, anoxic waters of these basins, bulk measurements showed an increase in concentration of some dissolved organic matter (DOM) components such as dissolved organic carbon (DOC) and dissolved organic sulfur (DOS). However, the direct abiotic effect of deoxygenation and oxygen variability into the DOM composition remains unclear. In this study, we combined state-of-art techniques in analytical chemistry, including Fourier-transform ion-cyclotron-resonance mass-spectrometry (FT-ICR-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES) and high temperature catalytic oxidation (HTCO), to quantitatively and qualitatively characterize the elemental (dissolved organic C, N, S and P) and molecular composition of DOM in three anoxic basins: the Mariager Fjord (Denmark, North Sea), the Gotland Basin (Baltic Sea), and the Black Sea. Samples were grouped in function of in situ oxygen concentration into three categories: oxic (>20 µM O₂), hypoxic (1 - 20 µM O₂) and anoxic (<1 µM O₂). In addition, we abiotically incubated samples from oxic-to-anoxic transition zone of the Gotland Basin (2.5, 55, 66, and 240 m depths) for 17 and 45 days at >200 µM and <1 µM O₂ concentration in the dark, continuously monitoring oxygen concentration by optical sensors inside a closed system previously flushed with N₂ air. Our results show that elemental composition of DOM follows similar vertical patterns in all three anoxic basins as a function of the different oxygen zonation. The highest concentration of DOS and dissolved organic nitrogen (DON) was detected in deep anoxic waters. In contrast, DOC and dissolved organic phosphorus (DOP) concentration was highest in oxic waters. At a molecular level, we identified a total of 8600 molecular formulas, mostly including CHO, CHON, and CHOS compounds. Largest dissimilarities (<53% Bray Curtis) were found in the DOM signature when comparing the three sites, particularly linked to aromatic and highly unsaturated compounds, suggesting specific autochthonous processes having a key role in shaping the DOM composition in each anoxic basin. However, the proportion of DOS-related molecular formulas increased under anoxic conditions at the three sites, especially in the deep, sulfidic waters of the Black Sea, pointing towards common abiotic processes playing a key role (e.g. DOM sulfurization). Furthermore, preliminary results of the abiotic incubation experiment revealed some degree of selectivity in the molecular formulas affected by abiotic exposure to oxygen. Namely, after 45 days being exposed to oxygen, a 5 – 16% of the total DOM showed differences in their intensities, being half of them DON, DOS and DOP molecular formulas. Our study reveals novel insights into DOM composition in anoxic basins and provides a conceptual framework for future studies investigating the impact of deoxygenation in the ocean.