Diversity and evolutionary history of cyanobacteria forming intracellular calcium carbonate

When ERC Calcyan started, we knew about one cyanobacterium only forming intracellular CaCO₃ (Couradeau et al., 2012) that we described for the first time and named Gloeomargarita lithophora (Moreira et al., 2017). By screening phylogenetically diverse cyanobacterial strains in culture collections, using scanning and transmission electron microscopies, we found 7 more strains forming intracellular CaCO₃ (Benzerara et al., 2014). Some of them had been studied in detail before by other groups but intracellular biomineralization had been overlooked. New sample preparation protocols have been key for this work. Interestingly, we also noticed that intracellular biomineralization of CaCO₃ was connected with cell division in all species within the monophyletic clade of Thermosynechococcus elongatus BP-1. In collaboration with Purificación López-García and David Moreira at ESE in Orsay, we designed specific primers against the 16S rRNA genes of several cyanobacterial groups forming intracellular CaCO₃ (Ragon et al., 2014). Based on this approach, we found a cosmopolitan distribution of cyanobacteria forming intracellular CaCO₃ and detected them in microbialites collected from four Mexican lakes, in karstic areas and in some thermophilic and hypersaline microbial mats collected in South America and/or Southern Europe. Recently, by carrying out phylogenomic analyses, we found that plastids in eukaryotes evolved from deep-branching cyanobacteria and that the present-day closest cultured relative of primary plastids is Gloeomargarita lithophora (Ponce-Toledo et al., 2017). This opens questions about the possibility that a transfer of CaCO3 biomineralization occurred from cyanobacteria to eukaryotes.