Spatial dynamics of marine dissolved organic matter in the Benguela upwelling system

Marine dissolved organic matter (DOM) is one of the largest active carbon pools on Earth, storing one thousand times more carbon than all living organisms in the ocean combined, and similar amounts of carbon than all CO₂ in the atmospheric reservoir. Coastal upwelling systems are extremely productive marine regions where surface waters mix with nutrient-rich waters from below, generating hotspots for carbon cycling with highly active microbial communities in the surface and low dissolved oxygen concentration in the deep waters. However, there is a lack of mechanistic understanding with respect to DOM accumulation in upwelling systems, and the sources and processing of DOM within these productive systems are an understudied topic. Here, we performed a novel spatial analysis of the DOM dynamics in the Benguela upwelling system off Namibia both regionally (from shelf to open ocean) and vertically (from surface ocean to subseafloor). To do so, we applied state-of-the-art molecular characterization of DOM using Fourier transform ion-cyclotron resonance mass spectrometry (FT-ICR-MS). By FT-ICR-MS, we identified 29,769 DOM molecular formulas from 75 water column and 8 pore water samples. Molecular DOM analyses were complemented by organic and inorganic quantitative geochemical data and microbial 16S rRNA gene-based diversity coupled with shotgun metagenomic analyses. Furthermore, we performed a set of incubation experiments onboard to test the biodegradability of DOM, with special focus on the dissolved organic sulfur compounds and potential genes involved in sulfur cycling. Preliminary results showed highest differences in the molecular analyses between the sediment pore water DOM and the water column DOM. Most of the variability in the DOM dataset could be explained by the high proportion of compounds containing heteroatoms different than oxygen in the sediments (N, S and P), while oxygen concentrations did not show a clear effect on the DOM molecular composition in the water column. In addition, metagenomic sequencing revealed that marker genes involved in sulfur oxidation and reduction such as periplasmic sulfur-oxidizing proteins (sox), dissimilatory sulfite reductase (dsr), reverse dissimilatory sulfite reductase (rDsr), and adenylyl-sulfate reductase (apr) present throughout the water column and subseafloor. Remarkably, the highest abundance of sulfur-reducing genes was observed in sulfidic sediments whereas sulfur-oxidation genes showed minimal differences in abundance along the water column profile. Further statistical analyses (in progress) will allow us to identify connections between the microbial biosphere and the chemical diversity of DOM, which will help to better understand the mechanisms of biodegradation and accumulation of DOM in coastal upwelling systems and deoxygenated regions.