Silica Biomineralization of Unicellular Microbes under Strongly Acidic Conditions

Silica biomineralization associated with unicellular microbes (Cyanidium caldarium) living in strongly acidic hot springs were observed by electron microscopy. The unicellular microbes form green biomats undergoing photosynthesis in Kamuiwakka Falls, Hokkaido, Japan. The hot-spring water is strongly acidic, with pH less than 2, and rich in S. Electron microscopy observations showed that the cell walls of unicellular microbes served as sites for nucleation of silica, polymerization of silicic acid and adhesion of colloidal silica. The precipitates formed an amorphous silica crust on the cell walls, which consist of granular silica spherules with a uniform size. The spherules commonly assimilate the cell walls. Data from X-ray diffraction and electron diffraction of the silica crusts reveal that the crusts are amorphous or of low crystallinity. Electron-dispersion X-ray spectroscopy also showed that the crusts are mainly composed of Si with traces of S and Cl. The unicellular microbes have a double-layer cell wall; therefore, silica crusts may form a double layer. FT–IR spectra of cells with and without silica crusts indicated that N–H, C=O and C–N–H bands were derived from peptides in cells, whereas the Si–O band was derived from silica crusts. Some models also are suggested on the interaction between cell wall and silica under strongly acidic conditions. Processes of silica biomineralization of unicellular microbes as described in this paper have profound implications for evolution of siliciferous microbes.