Biomineralization at hot springs and mineral springs, and their significance in relation to the Earth's history]

Recently, there is strong interest on microbe-mineral interactions. This is related also to recent expanded knowledges on extremely severe environments in which microbes live. Interaction between microbes and minerals contains biomineralization processes. Varieties of biomineralization products are found not only in various geologic materials and processes in the earth's history but also in present surface environments. Some hot springs represent such environments similar to those of unique and extremely severe environments for life. In this short review, the author briefly shows some examples of biomineralizations at some hot springs and mineral springs, Japan. In such environments, iron ore was formed and some varieties of growing stromatolites were found. The varieties of stromatolite are siliceous, calcic and manganese types. Cyanobacteria and the other bacteria are related to form the stromatolite structure. In the Gunma iron ore, sedimentary iron ores were mineralogically described in order to evaluate the role of microorganisms and plants in ore formation. The iron ore is composed of nanocrystalline goethite. Algal fossils are clearly preserved in some ores. Various products of biomineralization are found in the present pH 2-3, Fe2(+)- and SO4(2-)-rich streams. Bacterial precipitation had variations from amorphous Fe-P-(S) precipitates near the outlet of mineral spring, to Fe-P-S precipitates and to Fe-S-(P) precipitates. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. The Gunma Iron Ore can be said as Biologically Induced Iron Ore. At Onikobe and Akakura hot springs, growing stromatolites of siliceous and calcareous types, were found, respectively. At Onikobe, The stromatolites grow especially near the geyser. Cyanobacterial filaments in stromatolite were well preserved in the siliceous and calcic stromatolites. The filaments oriented in two directions which form the layered structures were found. At Yunokoya hot spring, black and brittle stromatolitic structures which were composed of amorphous Mn minerals are growing. The form of these structures are hemispherical. Many bacteria that were coated with amorphous Mn minerals were found on these structures. Furthermore, Precambrian (Proterozoic : Wittenoom-Chichester region, western Australia) manganese stromatolite was briefly shown in comparison. The black stromatolite has been clarified to be composed of todorokite. Small spotty and donuts-like shaped todorokite aggregates which are very similar to biologically induced Mn-precipitates were found in massive dolomite layers.