Structure and architecture of a stromatolite from a Mediterranean stream

The architecture of a riverine stromatolite (3 to 7 mm thick) was studied b y 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 stromatol ite. Cyanobacterial cells and filaments, mucopolysaccharides and void space s share that area. Filaments were present in the lowermost layer of the str omatolite, even though less densely arranged than in the upper layers. CLSM observations after staining with fluorescent probes (Concanavalin A) showe d 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 e xopolymer in the stromatolite may allow an extremely fast rewetting after d esiccation and its survival after droughts. Spaces not occupied by mucilage s and cells were much more abundant in the lower (ca 70 %) than in the uppe r layers (ca 20 %). The abundance of voids may be determinant of the diffus ivity and adsorption capacity within the stromatolite, allowing allocation of resources (gases and nutrients) in the lowermost areas of the stromatoli te. These structural characteristics help to understand the physiological a daptations observed in stromatolites inhabiting unfavourable environments.