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PhD Sorbonne University

Thesis in progress since October 2, 2023 (Doctoral School of Environmental Sciences of Île-de-France)
Thesis directors: Eva Ortega-Retuerta (LOMIC) and Pierre Galand (LECOB)

Assessing the influence of contrasting external organic matter inputs on ocean microbial communities: Mediterranean Sea vs. Arctic Ocean case studies.

Context At 662 Pg C, marine organic matter (OM) is one of the largest reservoirs of reduced carbon on Earth and largely impacts different biogeochemical processes in the ocean. The link between OM and microbes is reciprocal: While OM serves as a substrate for heterotrophic prokaryotes, affecting their diversity and metabolic potential, prokaryotes are key mediators in the formation, transformation, and storage of OM. While photosynthesis is the main OM source in the ocean, external OM inputs are increasing due to anthropogenic pressures. An example is the rise of wildfires, whose ashes are transported into the ocean and can act as an external source of nutrients to the ecosystem. The impacts of wildfire ashes on marine prokaryotes and OM, by contrast, have not yet been described. On the other hand, high latitude ecosystems such as the Arctic are particularly vulnerable to climate change. Summer sea ice decline and longer ice-free periods, added to increasing freshwater (river inflow, net precipitation and glaciers) and terrestrial (permafrost thaw and coastal erosion) inputs, will alter the OM field and thus the prokaryotes that grow on it. Objective The main objective of the PhD will be to explore the connections between OM composition from contrasting external sources (forest fires in the Mediterranean Sea, multiyear ice, glaciers and terrestrial sources in the Arctic Ocean) and prokaryotic metabolism and composition. The main questions to be resolved during the PhD will be: - How do wildfire affect prokaryotic activity and composition through the release of OM? - How do Arctic Ocean prokaryotic communities interplay with the OM composition of different external sources (multi-year ice, ice-associated phytoplankton blooms, glaciers) Approaches The PhD thesis objective will be achieved by both field characterizations and OM degradation experiments. To assess the effect of wildfire ashes on OM and prokaryotes (first chapter), the student will analyze a sample set collected during a mesocosm experiment performed (July 2022) using Mediterranean Seawater Mesocosms. For the Arctic Ocean work, the student will participate in the Refuge-Arctic project (PI Mathieu Ardyna, Takuvik) on a cruise planned in summer 2024 onboard the Canadian Icebreaker Amundsen. To assess the geographical distribution of the different OM sources and its link to prokaryotic communities (chapter 2), OM composition from different endmembers will be characterized (optical properties, amino acids, stoichiometry and molecular composition). While prokaryotes will be characterized by its respiration, enzyme activity rates and composition (16s sequencing). Finally, the effects of different OM sources on microbial communities will be studied experimentally using incubations with in situ collected microbes (chapter 3). Samples taken during the PhD will be analyzed at the different facilities hosted in Banyuls observatory: LOMIC laboratories, but also the BioPIC (imaging and flow cytometry) and Bio2Mar (chemistry and molecular biology) plattforms. DOM molecular composition will be analyzed using the Infraanalytics facility located at the University of Rouen. The PhD thesis will be done at the Laboratoire d'Océanographie Microbienne (LOMIC) in Banyuls sur Mer, Southern France. The LOMIC is composed of a diverse team of researchers with expertise in marine biogeochemistry, microbial ecology and diversity, and physiology of key microbial model organisms