Ultrastructures of cyanobacteria forming intracellular calcium carbonate

How CaCO₃ inclusions are arranged within cyanobacteria, how they precipitate and whether this takes place within an intracellular compartment remains mysterious. This is why we used a combination of microscopies to study the ultrastructure of cyanobacteria forming intracellular CaCO₃. Using conventional transmission electron microscopy, tomography, ultramicrotomy, and scanning transmission X-ray microscopy (STXM), we showed that many of these cyanobacteria form polyphosphate granules adjacent or engulfing Ca-carbonate inclusions (Li et al., 2016). Ca-carbonates were scattered within G. lithophora cells but were sometimes arranged in one or several chains. In contrast, Ca-carbonates formed at cell septa in S. calcipolaris suggesting a connection between biomineralization and cell division. We showed that conventional ultramicrotomy does not preserve Ca-carbonates explaining partly why intracellular calcification has been overlooked in the past. Recently, in collaboration with Martin Sachse in the Ultrapole research unit at Institut Pasteur, we managed to use Cryo-EM of vitreous sections (CEMOVIS) to provide unprecedented images of the genuine ultrastructure of these cyanobacteria, preserving CaCO₃ inclusions (Blondeau et al., in prep).

Cryo-transmission electron microscopy image of a cyanobacterium forming intracellular CaCO3. The cell was sectionned in vitreous ice.