Interactions between Dissolved Organic Matter and microorganisms: implications for the global carbon cycle

The ocean holds a huge amount of carbon in the form of Dissolved Organic Matter (DOM). Most of the DOM pool is recalcitrant, namely hard to manage or control, and persists in the oceans for years to millennia without being degraded by the microbial communities. The reasons behind the persistence of DOM in the ocean has challenged researchers for decades with the most recent hypotheses indicating that the recalcitrance depends on the complex interactions between microbial communities and DOM within an environmental context that can change in time and space. The main aim of this project is to explore the interactions between microorganisms and DOM and to highlight their role in establishing the biological lability of DOM as well as their implications for the global carbon cycle. To achieve this goal, laboratory and field experiments as well as an extensive elaboration of in-situ data were carried out using the Mediterranean Sea (MedSea) as a case study. In this Thesis, short-term incubations experiments have been used to: (1) investigate the release and the biological lability with respect to molecular weight and optical properties of phytoplankton DOM (Chapter 3), (2) test the hypothesis that different microbial communities can use the same pool of compounds with different removal rates (Chapter 4) and (3) study the microbial loop functioning and the DOM dynamics in the Eastern MedSea (EMed), with the main aim to constrain the reasons behind the accumulation of DOC (Chapter 5). The incubations using phytoplankton DOM provide new insights into the biological lability of algal exudates, indicating that molecules with different molecular weights are removed with different dynamics, even if, at the end of the incubations, their proportion remain stable, suggesting that the lability of DOM does not depend on its molecular weight. Our study indicates that, early in the incubation, high molecular weight proteins are preferentially hydrolyzed into small peptides. Protein degradation is accompanied by a change in the hydrophobicity of the degradation products. The exudation of a new class of fluorophores, associated with flavins, not frequently observed in the ocean, is also observed. Incubation experiments carried out in the EMed during the oceanographic cruise PERLE2 (February-March 2019), show that the intermediate microbial community sampled at 500 m depth rapidly consume up to 30% of the high molecular weight DOM (HMWDOM) accumulated in the surface layers, whereas the microbial community sampled from surface waters does not benefit from the HMWDOM addition. These results suggest that the metabolic capabilities are one factor that may determine the biological lability of DOM.