Nawal Bouchachi

In 2017 I was graduated as a marine environment engineer in Algeria. As I have a great passion for research in biogeochemistry and carbon cycling in marine ecosystems, I joined the marine sciences master II in Sorbonne university in 2018.

During this master I had the opportunity to be an intern in the LOMIC with Eva Ortega-Retuerta. The aim of the internship was to assess the effect of nutrient limitation on the composition of dissolved organic matter produced by marine heterotrophic bacteria.

In keeping with this study, I started my PhD thesis in October 2019 under the supervision of Eva Ortega-Retuerta and Ingrid Obernosterer. I work on the role of the microbial carbon pump in carbon cycling in the Mediterranean Sea. In fact, the ocean absorbs 30% of anthropogenic CO2, and it contains a vast amount of carbon in the form of oceanic dissolved organic matter (DOM; 660 Gt). However, many of the mechanisms that produce and recycle DOM in the ocean are still to be resolved, but essential to predict further changes in CO2 capture.

Heterotrophic bacteria play a key role in DOM processing in the ocean. Roughly 50% of carbon that is fixed by photosynthesis is channelled by them, either to produce biomass (bacterial production) or CO2 (respiration). Less known, however, is the role of bacterial as DOM sources. The so-called microbial carbon pump (MCP) states that bacteria produce refractory DOM, this is, compounds that resist further remineralization and are thus stored in the ocean for thousands of years. In oligotrophic ecosystems, such as the Mediterranean Sea, the MCP is predicted to play a major role in carbon sequestration. However, the ultimate reasons why this DOM is produced and which are the conditions that make it refractory remain unclear.

So, we aim to understand the role of the MCP on DOM dynamics in the Mediterranean Sea, an oligotrophic ecosystem highly vulnerable to climate change. In this marine system, an accumulation of dissolved organic carbon during the stratified period (end of summer) is uncoupled from phytoplankton productivity maxima (early spring). The ultimate source of this DOM (phytoplankton vs. bacterial) will impact its lability and thus its final fate in the ocean, that is, remineralized vs. accumulated, once entering the deep ocean by water mixing. This work combines: nutrients limitation experiments using bacterial model strains, monitoring the temporal dynamics of DOM quality in the open sea site MOLA in comparison to the variability of microbial activities and composition to elucidate the main sources of the different DOM pools.