About 98% of the ocean’s biomass is composed of microorganisms like the tiny algae, phytoplankton. Tiny but mighty when it comes at capturing carbon dioxide (CO2). Phytoplankton acts for half of the Earth’s photosynthesis, allowing ocean’s to supply major living resources and dioxygen (O2). Microbial respiration is the other fundamental biological process that counterbalances photosynthesis and returns organic carbon back as CO2. Yet, despite ocean’s pivotal role in global climate, microbial respiration remains one of the least explored metabolic processes; so that, whether oligotrophic ocean is a net sink or source of CO2, is highly debated for the last 20 years. The BULLE project aims to evaluate the ocean’s metabolic balance between photosynthesis and respiration by looking at the production of CO2 evolved to that O2 consumed by marine bacteria, the so-called “respiratory quotient” (RQ). Limits of detection of biological CO2 production have left RQ measurements far behind the multitude of investigations of photosynthesis. BULLE will face these challenges using the most recent technologies. The project strongly relies on the multidisciplinary expertise I will share with my host lab to tackle this issue at both cellular and community level. Specifically, BULLE aims to (1) investigate how the chemical characteristics of nutrients (Fe and C) regulate the RQ in bacterial cells and (2) study the links between the RQ, net primary production and bacterial activities. An innovative aspect of BULLE is the implementation of continuous measurements of O2/N2 and pCO2 concentrations respiration, and the deployment of In Situ Oxygen Dynamic Autosampler (IODA) instrumentation in the coastal NW Mediterranean Sea. The training I will receive with BULLE will help me give my career a new direction from a lab expertise towards high resolution in situ observations. In return, I will transfer my experience in microbial metabolisms and radioisotopes tracking methods to the host team.

The boreal forest is one of the most important global carbon sinks but its carbon fluxes and total amount of sequestered carbon depend on the regional climate variability. Because of this sensitivity to climate, boreal trees are also important natural archives of current and past climate change. During this project, we will use a data-model approach to improve our understanding of the links between forests and climate in a Canadian boreal region over the last millennium. More specifically, we will adapt the process-based ecophysiological model MAIDENiso to investigate factors influencing the growth and underlying biogeochemical processes of black spruce (Picea mariana (Mill.) B.S.P.), the most representative species of the North American boreal biome. This will give an insight into carbon storage in the taiga and will provide the first multiproxy (ring widths and δ18O and δ13C in tree-ring cellulose) regional climate reconstruction in Eastern North America over the last millennium. MAIDENiso will be calibrated on a recently developed network of tree-ring data from the taiga of Quebec. This project will have important implications. First, we will get information on the capacity of black spruce forests to adapt to climate change and to act as carbon sink using an innovative approach that can be transferred to study European boreal ecosystems as well. Second, we will reduce the uncertainties on the estimates of the climate variability of the last millennium in a region that has historically been under-represented in the Northern Hemisphere network of climate reconstructions (see IPCC AR5). Finally, we will be able to analyze the impact of each climate forcing (volcanism, solar activity, CO2 concentration) on the regional climate and carbon sink variability. An important aspect of this project is its multidisciplinarity. Climatology, geochemistry, dendrochronology, tree physiology and numerical modelling will be used linking together European and Canadian scientists.

The Mediterranean Sea is a climate-change hotspot, which is often considered a miniature model of the world ocean. Therefore, it may allow foreseeing some of the expected global effects of climate change. Sponges hold numerous key roles in benthic ecosystems functioning. In the Mediterranean Sea, they create complex habitats serving as spawning and nursery grounds for fish and invertebrates, and their filter-feeding activity affects entire food webs and bentho-pelagic couplings. These keystone organisms are frequently suffering disease outbreaks and mass mortalities that are mostly triggered by marine heatwaves. Sponges are well-known holobionts that host complex symbiotic microbial communities. While some members of these communities offer adaptive advantages, others may contribute to disease outbreaks when environmental conditions change. This interdisciplinary research aims to discover the complex mechanism of thermal-stress-related necrosis and mortality of the sponge holobiont by studying the effect of heatwaves on the Mediterranean bath sponge Spongia officinalis through a holistic approach that includes morphological, transcriptomic, metagenomic, and metabolomic responses of the holobiont. To study causative as well as correlative disease events, the experimental setting of this project will combine in situ sampling of healthy and diseased individuals, as well as exposure of individuals kept in aquaria to simulated marine heatwaves. These will provide the first insight, not only to the change in the microbial community composition but also to the functionality of its members during thermal stress. It is imperative to understand the mechanism of disease prevalence and mortality and to likely suggest ways to mitigate this phenomenon. The expected increasing frequency of marine heatwaves can cause irreversible damage to entire ecosystems that rely on sessile invertebrates such as sponges and thus to numerous ecosystem services provided to human societies.