Theses defended

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Constance CHOQUEL - 9th July

"Ecology of benthic foraminifera, geochemical and biological interactions: multidisciplinary approach at different scales"

The overall aim of this PhD thesis was to investigate sedimentary micro-environments and ecosystem functioning of two coastal areas. We combined different high spatial resolution methods and multivariate analyses at different spatio-temporal scales to reveal interactions between benthic faunal and geochemical compartments. Firstly, we investigated two stations with contrasted oxygen, nitrate and manganese conditions in the Gullmar Fjord (Sweden). We revealed the high contribution (50–100 %) of denitrifying benthic foraminifera to the nitrogen cycle in oxygenated and nitrate-rich micro–environments. Nitrogen and manganese cycles are closely related to oxygenation conditions of the ecosystem. Our results highlighted the high contribution (87 %) of macrofaunal bioirrigation to Mn release to the water column under hypoxic conditions. Secondly, we focused on a monthly monitoring of two ecological bioindicators groups; microphytobenthos (MPB) and foraminifera in the Bourgneuf Bay mudflat (France). We showed that foraminiferal reproduction events were modulated by unfavorable conditions (high hydrodynamic and winter conditions) versus favorable conditions (low hydrodynamic and summer conditions). We also demonstrated that foraminiferal species fed preferentially on diatom species based on their shape, size and life-forms. We further compared, with high spatial resolution methods, geochemical conditions at two contrasted months, which allowed to clarify the behavior of redox species and nutrients. Then, foraminiferal micro-distributions indicated the state of sediment instability versus stability. Finally, this doctoral research opens new perspectives in the use of high spatial resolution in 2D/3D to solve complex benthic ecology problems.

Sitraka RANDRIAMAMONJY - 17th March

"Role of bacterial siderophore in biogeochemical processes controlling the mobility of metals in vineyard soils and their transfer to the plant"

The continuous use of copper-based fungicide has led to the increase of total concentration of copper in most vineyards topsoil. This may present a risk of contamination because Cu residues accumulate in soils and remain in ecosystems for a long time after application. Some Cu-contaminated vineyards might therefore be remediated when re-used and assisted phytoextraction by siderophore producing bacteria seems to be the most suitable method. In this work, before being able to validate this hypothesis, it is important to understand the potential of pyoverdine (bacterial siderophore) and the effectiveness of the bacteria that produces it on copper mobility and phytoavailability as well as their effect on plant growth. Pyoverdine (Pvd) was therefore supplied to a collection of vineyard soils and results demonstrated that it’s has systematically increased the mobility of Cu in all soils and reduce Cu2+ concentration. From these results, the improvement of copper phytoavailability by pyoverdine was only true if the Cu-Pvd complex participated to copper uptake by plant. Which was demonstrated from a ligands-buffered solution experiment, where at the same free-copper activity, Cu uptake by sunflower was higher in solution added of Cu-Pvd complex than in absence. The results suggested that this contribution is made by increasing the diffusion flux towards the root with a possible dissociation of the complex around the root zone, thus leading to a better absorption of Cu by sunflower. Knowing that pyoverdine has the potential in increasing copper phytoavailability, we also tested to what extend the activities of a siderophore-producing bacteria could affect copper mobility and phytoavailability in vineyard soils and in presence of plant. Which was possible by using 2D DET and 2D DGT methods that permitted to highlight the distribution of the mobilization activities of Pseudomonas fluorescens in the rhizosphere of Helianthus annuus. This experience showed that the zone of mobilization of P. fluorescens was different according to the vineyard soils (non-carbonated vs. carbonated), which could explain why only sunflowers grown on the inoculated carbonated soil showed a significant difference in the concentration of Cu accumulated in the aerial parts. It has also been shown that siderophore plays a partial role in the mobilization of elements in the sunflower rhizosphere, suggesting other processes involved by P. fluorescens. In the last experiment we did not directly test an in situ bioaugmentation-assisted phytoextraction because we considered it necessary to first investigate the 2D distribution of mobility, phytoavailability and total concentration of Cu in an undisturbed and planted vineyard soil with wheat and sunflower, as well as the microbial diversity that may be present there.

Valentin JOLIVET - 27th January

"Incorporation of iodine in nuclear glasses by vitrification under high pressure"

129I is a product of the fission of uranium in nuclear power plants. It is radiotoxic, very mobile in the environment, and has a long half-life (15.6 Ma). The conditioning of 129I in nuclear glasses for geological storage is complicated by its high volatility at high temperature. The vitrification of nuclear glasses under pressure is a solution that can overcome this problem, because the solubility of volatile elements in liquids increases with pressure. Nuclear glass analogues have been vitrified under high pressure (0.5-2 GPa) to determine the solubility of iodine in glasses, as a function of thermodynamic and compositional parameters. The solubility of iodine in glasses depends on pressure, boron content, but also on the content of non-network forming cations. Polymerized borosilicate glasses such as ISG nuclear glass simulant incorporate ~1 mol.% iodine, while depolymerized glasses such as "Low Activity Waste Glass" incorporate ~2 mol.%. Iodine is incorporated into glasses in the vicinity of the non-network-forming cations. In doing so, it changes the polymerization state of the lattice. Iodine has a depolymerizing effect on polymerized glass, and vice versa for depolymerized glass. The solubility of iodine is also strongly influenced by its oxidation state, with I5+ being much more soluble in glass than I-.