Unveiling the control of N and P on DOM fate in a Mediterranean coastal environment

Abstract

Dissolved organic matter (DOM) and heterotrophic prokaryotes (HP) are key players in the oceanic carbon cycle. Although several biotic and abiotic factors controlling DOM fates are known, the hierarchy of their respective influences is still debated. Two contrasting Mediterranean coastal sites were sampled: a harbour under strong continental and anthropogenic influence (T) and an open coastal area (G). Interestingly, similar dissolved organic carbon (DOC) concentrations were observed in both samples. However, they showed marked differences in dissolved inorganic nitrogen and organic phosphorus concentrations (60-fold and 80% higher value in T), as well as in DOM optical properties and molecular composition. Incubation experiments were performed to expose the HP communities of each site to dissolved substances from T and G for three weeks. DOC removal was similar (-10 %) regardless HP origin and dissolved substances characteristics. HP growth and their maximal abundance were higher (+ 300 %) with dissolved substances from T, regardless HP origins. This indicates different fates of DOC processed by microbial communities as a function of abiotic determinants. Higher HP growth was associated to elevated initial content and higher consumption of inorganic nitrogen, organic phosphorus, three fluorescent DOM components, nitrogen-containing molecules and carbohydrates. These results provide insights into the main drivers of marine DOM fate: at similar DOC concentrations and low inorganic P concentrations. We evience the preferential consumption of lignin-like compounds where theoretically more labile molecules were available, thus reinforcing the need of in depth molecular studies for a better understanding of DOM-microbes interactions in the ocean.