Soutenance de thèse de Sitraka RANDRIAMAMONJY - "Role of bacterial siderophore in biogeochemical processes controlling the mobility of metals in vineyard soils and their transfer to the plant"

Mercredi 17 Mars 2021par Stéphanie BEAUNAY


Abstract: 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.

Sous la direction de Thierry LEBEAU, Professeur à l'Université de Nantes.