This PhD thesis is part of the project EXODIA (EXOpolymeric substances as catalyzers of DIAgenetic reactions: implications for metal accumulation in modern and ancient estuaries) funded by the Agence Nationale de la Recherche between 2024 and 2028. Estuarine biofilms have abundance of exopolymeric substances (EPS), predominantly produced by diatoms and bacteria. EPS are mainly composed of polysaccharides and proteins, and include reactive functional groups, such as carboxyl or sulfate. Reactive groups allow binding of EPS to mineral surfaces (e.g, clay), changing sediment properties, and have the potential to bind large quantities of metals. Clay-EPS complexes have a strong potential for heavy metal immobilization by providing a large variety of complexation sites. EXODIA aims at characterizing the capacity of EPS extracted from sediments and of EPS produced in culture to form complexes with metals, ultimately leading to their precipitation, and the transformations of EPS and minerals (precipitation vs. dissolution) during sediment burial and diagenesis.

The objectives of this PhD thesis will be: (i) a characterization of exopolymeric substances (EPS), microbial activity, and of metals and minerals within porewater and sediments in two estuaries with contrasted climate: the Gironde Estuary (Southwest France), and the Guadiana Estuary (at the border between Portugal and Spain). Such analyses will be performed on sediment cores (>5 m in length); (ii) to understand the role of EPS in biogeochemical cycles, and particularly in the precipitation and dissolution of metals during diagenesis; (iii) to cultivate benthic diatom biofilms, extract and characterize their EPS in various conditions (salinity , sedimentary substratum, metal concentrations); (iv) to use the produced EPS in experimental diagenesis experiments.

This PhD project will be part of EXODIA ’s Task 1 “Relationships between microbial activity , EPS, sediment, metal and water properties in modern estuaries” and Task 2 “Culture, selection and identification of key organisms involved in EPS turnover and metal cycling”. A strong interaction is expected between the PhD candidate and a second PhD student who will cultivate microorganisms in oxic and anoxic conditions in Task 2.