Participation of a cyanobacterial S layer in fine-grain mineral formation

Cyanobacteria belonging to the Synechococcus group are ubiquitous inhabitants of diverse marine and freshwater environments. Through interactions with the soluble constituents of their aqueous habitats, they inevitably affect the chemistry of the waters they inhabit. Synechococcus strain GL24 was isolated from Fayetteville Green Lake, New York, where it has a demonstrated role in the formation of calcitic minerals. In order to understand the detailed interactions which lead to mineral formation by this organism, we have undertaken detailed ultrastructural studies of its cell surface and the initial events in mineral growth using a variety of electron microscopic and computer image enhancement techniques. Synechococcus strain GL24 has a hexagonally symmetrical S layer as its outermost cell surface component. The constituent protein(s) of this structure appears as a double band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with M(r)s of 104,000 and 109,000. We demonstrate that the S layer acts as a template for fine-grain gypsum and calcite formation by providing discrete, regularly arranged nucleation sites for the critical initial events in the mineralization process. To our knowledge, this is the first time that a bacterial S layer has been shown to have a role in mineral formation in a natural environment, and this report provides conclusive evidence for the specific involvement of bacterial surfaces in natural mineral formation processes.