Structure and architecture of a stromatolite from a Mediterranean stream

The architecture of a riverine stromatolite (3 to 7 mm thick) was studied by means of scanning electron microscopy (SEM) of thin sections and confocal laser scanning microscopy (CLSM). The simultaneous use of the 2 techniques showed that the stromatolite is a highly porous structure, where ca 50% of the space is free of carbonated material. That area was slightly higher in the upper (46 to 57%) than in the lower layer (39 to 44%) of the stromatolite. Cyanobacterial cells and filaments, mucopolysaccharides and void spaces share that area. Filaments were present in the lowermost layer of the stromatolite, even though less densely arranged than in the upper layers. CLSM observations after staining with fluorescent probes (Concanavalin A) showed the existence of a huge network of exopolymers, mainly in the upper part of the structure. Cyanobacterial filaments were less abundant than mucilage in the lower layer (8.9 vs 17.6%), this difference being more moderate in the upper layer (45 vs 33%) of the stromatolite. The extensiveness of the exopolymer in the stromatolite may allow an extremely fast rewetting after desiccation and its survival after droughts. Spaces not occupied by mucilages and cells were much more abundant in the lower (ca 70 %) than in the upper layers (ca 20 %). The abundance of voids may be determinant of the diffusivity and adsorption capacity within the stromatolite, allowing allocation of resources (gases and nutrients) in the lowermost areas of the stromatolite. These structural characteristics help to understand the physiological adaptations observed in stromatolites inhabiting unfavourable environments.