Yihua HE Thesis defense

Diatoms are ubiquitous unicellular microalgae that are commonly used as bioindicators to evaluate the impact of contaminations on the ecological health of aquatic ecosystems. In recent decades, diatoms have received increasing attention as a decontamination material for removing heavy metals from contaminated effluents or for in situ bioremediation purposes. However, the interactions between diatoms and radioelements, e.g., uranium (U) and radium (Ra), remain relatively unclear and poorly documented. Therefore, this work aims to study, at both the macroscopic and molecular level, the interaction of U and Ra with a xenic Achnanthidium saprophilum diatom culture in which a naturally occurring bacterial community is symbiotically associated to diatoms. Batch-type U and Ra bio-association experiments are performed to evaluate the adsorbed and incorporated fractions of U/Ra in the diatom culture. Besides, various microscopic and spectroscopic techniques are applied to investigate the localization and speciation of U at the cellular level. Results demonstrate the significant bioassociation of U and Ra with the diatoms and highlight the important role of carboxylic and phosphate groups in the U-diatoms interaction. Both adsorption and incorporation are observed for U and Ra in the diatom culture, with their distribution depending on the diatom growth phase and on the contact time. This work contributes to a better understanding of the interaction of U and Ra with the xenic diatom culture and also highlights the contribution of the bacteria symbiotically associated with the diatoms to the overall interactions.