Textures and processes of laminated travertines formed by unicellular cyanobacteria in myoken hot spring, southwestern Japan

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.