Textures of modern lacustrine stromatolites on Kiritimati (Line Islands, Central Pacific Ocean), and of buried layers in the stromatolitic carbonate sediments from French Polynesian atoll lakes ( kopara ), have been studied using cryo-scanning electron microscopy (SEM equipped with a freeze-drying sample preparation device). This study confirms that microscopic three-dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria, and from other extracellular polymer secretions, are common components of microbial sediments, of which they may form the framework, In addition to this role in sediment cohesion and formation of microstructure, the organic framework appears to be involved in carbonate precipitation within the stromatolites, through chemical (nucleating), steric, and hydrodynamical controls. The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites. Bacterial, including nannobacterial, carbonate bodies, and carbonate-impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied. All these carbonate precipitation processes may cooperate in lithification of a given sediment.

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