To elucidate the role of aluminum for silica deposition on the surface of microbe, hot spring water and siliceous deposit samples were collected along a stream in the Steep Cone in Yellowstone National Park, USA and were characterized. The SiO2 content of siliceous deposit samples ranged from 71.86% to 93.68% and the content increased along the stream. Meanwhile, the Al2O3 content decreased inversely 6.27% to 0.18% depending on the Al concentration in the hot spring water at each sampling point. Most of the aluminum in the siliceous deposit sample collected at the springhead was present as 4-coordinated Al (Al(4)), showing that the deposit was formed by inorganic reactions between monosilicic acid and aluminum ions. The proportion of 6-coordinated Al (Al(6)) of the siliceous deposit samples increased downstream. As aluminum ions combined on the surface of microbes are present as 6-coordinated Al, it is suggested that aluminum ions are preferentially combined with functional groups on the surface of microbes, and then silicic acids are chemically adsorbed to the aluminum ions that play an essential role as active sites of deposition of monosilicic acid from hot spring water.
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