The IRONWOMAN project aim to validate the hypothesis of the primordial role of marine iron-oxidizing bacteria (FeOB) in promoting the development of iron-rich mats, and impacting the iron biogeochemical cycle and the primary production in the deep oceans, according to the variations of the environmental conditions at play. For this purpose, we propose to carry out 1) in situ sampling of iron-rich mats through either punctual annual sampling or deployments of colonization experiments, and a newly developed nucleic acids and fluids sampler instrument enabling monthly collection; 2) continuous monitoring of physico-chemical environmental conditions at two contrasted deep-ocean environments of EMSO Azores (Atlantic hydrothermal site) and EMSO West Ligure (Mediterranean deep coastal plain), taking advantage of their status of deep-sea observatory (IR EMSO France); and 3) geochemical, isotopic, mineralogical, cultural and microbiological multi-omics analyses. Through these multidisciplinary and long-term approaches and instrumental development, the IRONWOMAN project would have an impact on better knowledge of ocean microbial biodiversity and its response to global environmental changes that could impact dO2 and dFe in deep ocean. To achieve our research objective, the work plan will be carried out by a consortium of complementary research team from four national institutions (MIO Marseille, GET Toulouse, URA-OMP Toulouse and LGE Marne La Vallée). The project is divided into five work packages: WP0-Management, to ensure the coordination between partner and the results dissemination; WP1-Sampling and Instrumentation, to ensure sampling and in situ experiments during the annual cruises but also the development of the FLUICS instrument; WP2-Characterization of environmental conditions to characterize the physico-chemical conditions surrounding iron-rich microbial mats; WP3-Biological characterization of mats, to characterize the microbial composition present in iron-rich mats, the active microbial species and their functioning via OMICS approaches; and WP4-Ex situ enrichment culture of microbial mats, to investigate the influence of dFe and dO2 variations on the functioning of microbial mats and iron acquisition pathways. The IRONWOMAN project will be the first dedicated multidisciplinary and long-term approach (relying on TGIR FOF and IR EMSO-France, the French node of the European infrastructure EMSO which is a legal entity under European law ERIC) conducted on entire microbial mats, leading to a full coverage of the complex interactions between them and their environment. Therefore, a combined strategy between in situ colonization through the development of a new device FLUICS and in vitro cultures, will allow us to improve our knowledge on the formation and evolution of iron-rich microbial mats with regards to environmental forcing. Through this topic, The IRONWOMAN project enters within the ANR research axis 1.2 “Terre Vivante”. Indeed, it addresses part of the objectives of the United Nations Ocean Decade (2021-2030) by developing a better knowledge and understanding of the ocean in order to protect and restore the ecosystem and biodiversity. Furthermore, by developing the FLUICS instrument, disseminating our results and dropping off the physico-chemical and sequencing data at databases, it will contribute to the expansion of the global ocean observing system, another objective of the Decade of the Oceans. This project will provide data to define the role of FeOBs as an actor in the iron cycle for primary production in deep waters, on two deep marine sites with different environmental conditions, and the interaction between iron, carbon and nitrogen cycles inside the mats. From its title (IRONWOMAN) to the organization of its Consortium and Work Plan, the IRONWOMAN project is fully gender-sensitive. A total budget of €552k is requested for 48 months, including 60 months of staff support, divided between the partners.