The role of microbes in accretion, lamination and early lithification of modern marine stromatolites

For three billion years, before the Cambrian diversification of life, lamin ated carbonate build-ups called stromatolites were widespread in shallow ma rine seas(1,2). These ancient structures are generally thought to be microb ial in origin and potentially preserve evidence of the Earth's earliest bio sphere(1-3). Despite their evolutionary significance, little is known about stromatolite formation, especially the relative roles of microbial and env ironmental factors in stromatolite accretion(1,3). Here we show that growth of modern marine stromatolites represents a dynamic balance between sedime ntation and intermittent lithification of cyanobacterial mats. Periods of r apid sediment accretion, during which stromatolite surfaces are dominated b y pioneer communities of gliding filamentous cyanobacteria, alternate with hiatal intervals. These discontinuities in sedimentation are characterized by development of surface films of exopolymer and subsequent heterotrophic bacterial decomposition, forming thin crusts of microcrystalline carbonate. During prolonged hiatal periods, climax communities develop, which include endolithic coccoid cyanobacteria. These coccoids modify the sediment, form ing thicker lithified laminae. Preservation of lithified layers at depth cr eates millimetre-scale lamination. This simple model of modern marine strom atolite growth may be applicable to ancient stromatolites.