Keywords:spherulite, ACC, CaCO3, nucleation, hot spring, geomicrobiology
Carbonate spherulites are spherical particles of around 10 μm–1000 mm in diameter. Spherulites are formed e.g. in hot springs precipitating CaCO3, and gather attention because they are found from huge oil reservoirs. Although some researchers suggest microbial involvement, their detail formation process is still the matter of controversy. The present study, therefore, investigated spherulites formed in the Nagayu hot spring in Oita Pref. for elucidating their formation process. First, a cyanobacterial mat developing on the flow path of hot spring water was collected, embedded in a resin, and a thin section was prepared. Then, ca. 100 nm-thin foil section was made using a focused ion beam (FIB) apparatus, and observed using scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM). The results indicated that the investigated spherulite was composed mainly of aragonite, but amorphous calcium carbonate (ACC) and calcite were also recognized in the central part. In particular, the distribution of ACC appeared like microbial cells. From these observations, following formation process of spherulite was suggested: 1) ACC was formed on negatively-charged surface of microbial cells and/or their metabolic products, 2) ACC acted as a precursor of calcite, 3) aragonite was formed around calcite due to high Mg/Ca ration of hot spring water. On the other hand, cyanobacteria surrounding the spherulite were almost free from calcification, indicating that the CaCO3 nucleation sites were strongly influenced by physicochemical properties of substrates even in significantly high supersaturation.