EXOpolymeric substances as catalyzers of DIAgenetic reactions: implications for metal accumulation in modern and ancient estuaries – EXODIA

Estuaries are highly productive environments where the transition from freshwater to saltwater leads to increased metal availability and accumulation in the sediment. As such, estuaries are strategic natural laboratories to study the interaction between biofilms, sediments, water and metals. Estuarine biofilms have abundance of exopolymeric substances (EPS), predominantly produced by diatoms and bacteria. The mucilaginous matrix of polymers supports attachment of microorganisms to the sediment, protecting them against desiccation and pollutants. Reactive groups allow binding of EPS to mineral surfaces and also have the potential to bind large quantities of metals. We recently found that EPS can be preserved at high concentrations several meters deep in estuarine sediments. Part of the estuarine EPS (i) may resist microbial and early diagenetic alterations and (ii) form organo-mineral complexes with metals and detrital clay minerals that potentially increase EPS preservation and metal binding. However, EPS-metal interactions are underexplored in subsurface sediments. Using sedimentary cores, EXODIA proposes to characterize the fate of EPS-metal complexes several meters below the sediment-water interface.
The first objective of EXODIA is to understand microbial metabolisms related to EPS turnover in estuarine sediments. This will be achieved by characterizing the changes in sediment and porewater composition, microbial activity and EPS in function of depth. The second objective is to isolate and cultivate EPS producers and degraders from key sedimentary and/or geochemical interfaces. Following spectrometric or genomic identification, the representativeness of these isolates will be checked genotypically through a metabarcoding of the sedimentary microbial communities.
Modifications of metal availability by reactive EPS could play a major role in clay mineral diagenesis. The third objective is to analyze the capacity of EPS extracted from sediments and of EPS produced in culture to form complexes with metals, ultimately leading to their precipitation. High-resolution microscopy and spectroscopy will allow to quantify and map metals and their speciation within EPS. We will measure the affinity of natural EPS and that from cultures for metals using e.g., calorimetry and titration experiments. Seven metal species will be investigated: Ca, Mg, Fe, Mn, Zn, Cd and Cu. The fourth objective is to characterize the transformations of EPS and minerals in forced diagenesis experiments reproducing the conditions encountered by the sediment during burial in a sedimentary basin. For the first time, EXODIA will monitor the evolution of EPS-metal-clay complexes in diagenetic conditions. Using high-pressure and high-temperature reactors, we expect to observe the precipitation of authigenic phases similar to the ones documented in ancient estuarine rocks. The fifth objective is to search for biosignatures of EPS-metal interactions in a collection of ancient estuarine sandstones of increasing age, burial and diagenesis. In ancient sandstones, we have documented several early diagenetic mineral phases, which could have precipitated following EPS-mineral-porewater reactions. EXODIA will look for remnants of EPS or EPS-metal interactions within e.g., clay coats, using high resolution techniques (e.g., SIMS). We propose to characterize modern EPS molecules and their degradation products through diagenesis experiments. This unique approach will guide our strategy for the exploration of ancient EPS molecules. To this aim, we will use innovative techniques that will enable us to work with very small quantities of material. The results of EXODIA could help to reconstruct EPS-metal biogeochemical cycles in modern and ancient estuaries, with potential applications in bioremediation.