Séminaire

The oceans play a key role in absorbing carbon dioxide (CO2) emitted by human activities. This absorbing power depends largely on the
photosynthetic activity of phytoplankton, which take up CO2 using nutrients and sunlight. The productivity of phytoplankton is however strongly limited by the availability of nitrogen. In open ocean areas nitrogen is mainly provided through atmospheric nitrogen (N2) fixation, a process performed by specific microorganisms called diazotrophs. My previous work has been devoted to investigate biological N2 fixation in the ocean.

During my PhD at the University of Las Palmas de Gran Canaria (Spain) I investigated the magnitude and controls of N2 fixation in the North Atlantic. The results of this research showed that the Northeast Atlantic harbors a contrasted N2-fixing microorganism assemblage as compared to its western counterpart, and that it is strongly influenced by hydrography, nutrient fields and the deposition of Saharan dust. Remarkably, it was found that a substantial part of the fixed nitrogen is released out of the cell as organic matter. Marine organic matter is a complex pool of compounds that contains as much carbon as the Earth's atmosphere in the form of carbon dioxide (CO2). Thus, the dynamics of marine organic matter are bound to have important consequences for global climate. Intrigued by the interactions between organic matter and N2-fixing microbes, during my postdoctoral research I started investigating how organic matter shapes their distribution, activity and diversity in the Southwest Pacific Ocean. The results obtained so far reveal a previously unknown connection between organic matter and N2-fixers in the mesopelagic layer, which suggests that the magnitude of N2 fixation is much greater than recognized before and takes place in poorly studied areas of the ocean. This has direct implications in the magnitude of the global ocean nitrogen reservoir, and hence on the biological uptake of CO2 and the capacity of the oceans to regulate climate through biological production. The objective of this seminar is to present my research and myself to the members of LOCEAN, to exchange ideas and receive feedback, with the ultimate aim of promoting my candidature for a CR2 position at IRD (Concours chercheurs 2015). I propose a project targeted at studying the microbial transformation of organic matter into refractory compounds in the waters around New Caledonia, as a means of CO2 sequestration besides -or complementary to- the classical 'biological carbon pump'. The interaction between microbes and organic matter is a novel field that requires the application of integrated approaches. I propose a dual approach including organic geochemistry and molecular biology techniques to unveil the role of microbes in transforming organic matter. This approach can be further applied to other ecosystems and specific environmental issues, offering development options in IRD implantations other than New Caledonia, training and education opportunities, as well as bright future research perspectives. Finally, the foreseen integration into the IRD landscape and future research applications will also be presented.