Aggregation Phenomena in Cyanobacterial, Stromatolite Analogues

Earth’s biological history is largely the story of microbial evolution. If one is to understand the evolution of such fundamental processes as photosynthesis, it is imperative to recognize and interpret microbial fossils. To these ends, we examine the forces that shape a modern cyanobacterial mat from the hot springs of Yellowstone that is thought to grow in a manner similar to an ancient structure called a conical stromatolite. Here we show that the initial stages in the growth of this mat is analogous to crystal-nucleation with long-range interactions. This result provides intuition for the forces that shape modern mats and, by analogy, conical stromatolites. Furthermore, the importance of long-range interactions suggests that many current models of stromatolite morphogenesis, which rely exclusively on local forcing, are not applicable to the early stages of growth. To understand the evolution of life on early Earth, one must distinguish and interpret biotic and abiotic processes preserved in the rock record. Although there is often chemical evidence that signals the presence of life during the formation of a geological feature, many of the oldest presumed fossils are only identified by a distinctive morphology [8]. In such cases, how does one interpret a presumed fossil? One example that is abundant throughout Archean and Proterozoic formations is the conical stromatolite. These laminated, lithified sedimentary structures have long been thought to have formed when minerals precipitated within a microbial mat. The conical morphology of these deposits is thought to reflect the shape the microbial mat that grew it [1][4][5]. Assuming that these stromatolites are fossils, what does a conical morphology of a microbial mat imply about the types of microbes that grew it? In order to understand why a microbial mat might develop a conical morphology, we study a population of modern, cone-forming cyanobacterial mats from the hot springs of Yellowstone National Park. These mats grow in springs that are supersaturated in silica or calcium carbonate; as a result cones are mineralized in-situ. As these mats grow, they produce laminated mineral-precipitates which passively preserve the mat’s morphology. Since these mats grow in a manner similar to how some ancient stromatolites are believed to have formed, modern cone-forming mats offer a unique opportunity to test hypotheses about stromatolite morphogenesis [9]. In this paper, we examine the earliest