Organomineralization of Cyanobacteria Cell Envelopes: Insights from Laboratory Studies and Implications to Carbonates Nucleation, Mineralogy and Carbon Cycling

The organomineralization of cyanobacterial cell envelopes is a widespread phenomenon. Yet, the factors that trigger the nucleation and define the morphology of the minerals formed on cyanobacteria surfaces remain unclear. This dissertation explores the role of the surface chemistry of different cyanobacteria in the formation of calcium and magnesium carbonates. The main goal was to identify characteristics inherent to cyanobacterial cell envelopes that control the nucleation, the chemistry and the structure of carbonates minerals. Studies with marine and freshwater Synechococcus strains showed that the relative proportion of carboxyl to phosphoryl groups in different species regulates the kinetics and morphology of the carbonates. Moreover, the cell envelope composition appears also to vary as a function of the phosphorus concentration in the culture media. These variations cause not only changes in the surface reactivity but were shown to have an effective impact on the organomineralization capacity of Synechococcus. The impact of cyanobacterial extracellular polymeric substances (EPS) in the formation of proto-dolomite, a precursor of dolomite, in sabkhas sediments was also explored in this dissertation. Raman spectroscopy mapping was first used to show that the cyanobacterial EPS molecules and dolomite are spatially associated in the sediments. Solutions seeded and non-seeded with cyanobacterial EPS and exposed to different temperatures (25ยบC and 40ยบC) suggest that dumbbells and spheroids, morphologies often identified as proto-dolomite, formed only in the presence of EPS. This mechanism may be supported by the rich content in carboxyl functional groups and polysaccharides in EPS. This work provides evidence for how the surface reactivity of cyanobacteria promotes the fast precipitation of carbonates and controls the morphology of the precipitates often without any contribution from photosynthesis. Moreover, it recognizes the importance of characterizing microbial molecules in sediment in order to significantly improve our understanding of carbonate precipitation dynamics in modern environments.Ph.D