The bioavailability of U for plants is dependent on its speciation in vegetated soil. Natural organic matter (OM) is involved in the control of U speciation in wetlands and vegetated soils, favoring its mobilization (formation of aqueous metallo-organic complexes) or its immobilization (co-adsorption with OM at the surface of minerals). These OM are supramolecular assemblages (of hundreds of organic constituents of different natures) whose compositions and reactivities towards metals and minerals vary within a soil profile. This variability can be a function of bacterial communities (degradation of OM) and / or the preferential adsorption of certain constituents of OM at mineral-solution interfaces. In particular, the impact of microorganisms on the U enrichment of vegetated soils in wetlands remains to be clarified. The hypotheses to be tested are the indirect control of bacterial communities on U speciation, i.e. via the degradation of OM (whether or not followed by the sorption of organic species with a strong affinity for U) or direct control (bio-sorption, -accumulation or -mineralization). The objective of this thesis is to identify the role of microorganisms from vegetated soils of the Rophin wetlands (Puy de Dôme) on the speciation (aqueous and surface) of U, in particular on the organic speciation of U involving complex OM in soils, and their impact on the lability and bio-availability of U for plants. Incubation experiments in the laboratory will be carried out under conditions favoring microbial activity and therefore the degradation of organic matter on (i) “simplified” systems making it possible to specifically study MO-mineral-bacteria-U interactions and on (ii) soil samples taken from the vegetated surface horizon of the wetlands. The so-called “simplified” systems will be binary or ternary mixtures composed of a mineral, hematite, taken as a model of iron oxy-hydroxides, -which are omnipresent in the vegetated soils of the Rophin wetlands-, MO extracted (and purified) from the collected soil, and from a bacterial consortium enriched from this same soil, to which we will add U (VI). For each systems, we will characterize the distribution of U between aqueous phases, solid (MO-hematite / soil) and bacteria. The speciation of U in solution or at the surface of hematite will be characterized by a panel of complementary methods (SEC, ESI-FTMS, ATR-FTIR). At the same time, OM will be characterized by spectroscopic methods (UV, fluorescence) and also at the molecular scale (ESI-FTMS). The bacterial populations will be followed during the incubations by metabarcoding of the 16S gene. Finally, the bioavailability of U will be evaluated using a RHIZOtest® type device (ISO 16198 standard). This biotest will be carried out before and after the incubation of the “raw” soil sample, but also on the binary and ternary model systems (MO-bacterium-(hematite)-U(VI)) studied, which will then be mixed with inert silica in order to form a solid substrate in an amount compatible with the use of this device. This research using both mechanistic and operational approaches, and advanced techniques of chemistry and microbiology, will make it possible to clarify the role of microorganism-OM interactions on the speciation and bioavailability of U and to apply this knowledge to interpretation of measurements carried out on the vegetated soils of Rophin.