This interdisciplinary proposal stems from our recent discovery of deep-branching cyanobacteria that form intracellular Ca-Mg-Sr-Ba carbonates. So far, calcification by cyanobacteria was considered as exclusively extracellular, hence dependent on external conditions. The existence of intracellularly calcifying cyanobacteria may thus deeply modify our view on the role of cyanobacteria in the formation of modern and past carbonate deposits and the degree of control they achieve on this geochemically significant process. Moreover, since these cyanobacteria concentrate selectively Sr and Ba over Ca, it suggests the existence of processes that can alter the message conveyed by proxies such as Sr/Ca ratios in carbonates, classically used for paleoenvironmental reconstruction. Finally, such a biomineralization process, if globally significant may impact our view of how an ecosystem responds to external CO2 changes in particular by affecting most likely a key parameter such as the balance between organic carbon fixed by photosynthesis and inorganic carbon fixed by CaCO3 precipitation.Here, I aim to bring a qualitative jump in the understanding of this process. The core of this project is to provide a detailed picture of intracellular calcification by cyanobacteria. This will be achieved by studying laboratory cultures of cyanobacteria, field samples of modern calcifying biofilms and ancient microbialites. Diverse tools from molecular biology, biochemistry, mineralogy and geochemistry will be used. Altogether these techniques will help unveiling the molecular and mineralogical mechanisms involved in cyanobacterial intracellular calcification, assessing the phylogenetic diversity of these cyanobacteria and the preservability of their traces in ancient rocks. My goal is to establish a unique expertise in the study of calcification by cyanobacteria, the scope of which can be developed and broadened in the future for the study of interactions between life and minerals.