Stromatolites may be Earth's oldest macroscopic fossils; however, it remains controversial what, if any, biological processes are recorded in their morphology. Although the biological interpretation of many stromatolite morphologies is confounded by the influence of sedimentation, conical stromatolites form in the absence of sedimentation and are, therefore, considered to be the most robust records of biophysical processes. A qualitative similarity between conical stromatolites and some modern microbial mats suggests a photosynthetic origin for ancient stromatolites. To better understand and interpret ancient fossils, we seek a quantitative relationship between the geometry of conical stromatolites and the biophysical processes that control their growth. We note that all modern conical stromatolites and many that formed in the last 2.8 billion years display a characteristic centimeter-scale spacing between neighboring structures. To understand this prominentbut hitherto uninterpreted-organization, we consider the role of diffusion in mediating competition between stromatolites. Having confirmed this model through laboratory experiments and field observation, we find that organization of a field of stromatolites is set by a diffusive time scale over which individual structures compete for nutrients, thus linking form to physiology. The centimeter-scale spacing between modern and ancient stromatolites corresponds to a rhythmically fluctuating metabolism with a period of approximately 20 hr. The correspondence between the observed spacing and the day length provides quantitative support for the photosynthetic origin of conical stromatolites throughout geologic time.geobiology | photosynthesis | cyanobacteria | microbialite S tromatolites-attached, laminated, lithified sedimentary rocks accreting from a point or limited surface (1)-are commonly thought to record microbial interactions with sediments as old as 3.4 billion years (Ga) (2-4). In general, stromatolites are complex products of physical, chemical, and biological processes. Consequently, their shapes and textures may tell us little about specific microbial metabolisms. Past biological activity is, however, thought to be a prerequisite for the formation of a number of conical stromatolites that grew in quiet conditions, in the apparent absence of sediment, and in the presence of fast lithification (5-7).Assuming that small conical stromatolites were built by microbial communities throughout geologic history, one is led to ask what biological processes are implied by the presence of these structures. Notably, all modern conical stromatolites form in the presence of filamentous cyanobacteria growing under precipitating conditions without sedimentation (8-10). Because these and similar photosynthetic communities form cones even in the absence of lithification (8,(11)(12)(13), this morphology must arise from biological processes. The qualitative similarity between the shape of modener and ancient stromatolites has led to the hypothesis that ancient cones were al...

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