Biomineralization of pisoliths in hot springs

Biological activity can produce complex patterned structures in accretionary carbonate spheres (pisoliths) from hot springs. Pisoliths from a few millimeters to 50 mm diameter were collected from a geyser at the high-salinity Arima Hot Springs near Kobe, Japan. High-resolution electron microscope images show that microorganisms produced concentric laminar layers of aragonite alternating with Si- and Fe-rich layers. STEM elemental distribution maps show that the cementation of aragonite is associated with NaCl-rich bacterial cells and elevated phosphorous and sulfur concentrations. The filamentous microbes build the concentric framework for mineral laminae with a web-like network of microbial strands. The intricate patterns of mineralogical and bacterial variation in the pisoliths correlate with the change of water chemistry in the hot spring. These patterns could improve our understanding of nano-scale biomineralization. In addition, the terrestrial hot spring pisoliths might be a counterpart of the tiny spherules recently found on Mars.