Lamina-forming processes of moderately thermophilic (∼55°C) unicellular cyanobacteria were identified for the carbonate travertine in the Myoken Hot Spring, Kagoshima Prefecture, southwestern Japan. Continuous 28-h monitoring of surface textures clearly showed a diurnal pattern in sub-millimeter order lamination, comprised of daytime dark colored aragonite layers, and night-time light colored calcite layers. The layering was controlled principally by daily cycles of unicellular cyanobacteria closely related to Thermosynechococcus elongatusBP-1. During a day, cyanobacteria migrated to the surface, and formed a dark green biofilm, in which CaCO3 is precipitated as radial aggregates of needle aragonite. After sunset, calcite precipitation formed dendritic aggregates of rhombic crystals that cover the biofilm. Physicochemical conditions of the ambient water were stable throughout a day; therefore CaCO3 polymorphs (aragonite/calcite) were controlled by the presence/absence of certain microbial effects, such as extracellular polymeric substances secreted by cyanobacteria. When travertine growth reached the water level, a yellow-green microbial mat developed on the subaerially exposed surface. Here, filamentous cyanobacteria joined in the microbial community under lowered temperature (∼40°C). Aragonite crystal precipitation occurring in the microbial mat supported the microbial effects on CaCO3 polymorphs. Geomicrobiological processes, effects, and the environmental conditions demonstrated in this travertine provide insights into a further understand of microbial textures in ancient carbonate sediments.
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