Characterization of silica-induced protein and mechanism of microbial silica deposition

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A combined use of molecular ecological techniques and geochemical surveys revealed that thermophilic or hyperthermophilic microorganisms living in the geothermal environments are likely to be implicated in the formation of biogenic siliceous deposits. Electron microscopic observations indicated that numerous microorganism-like fabrics were presented in naturally occurring siliceous deposits such as siliceous sinter, geyserite, and silica scale, which suggests microbial contribution to silica precipitation. Molecular phylogenetic analysis suggested that extreme thermophilic bacteria within the genera Thermus and Hydrogenobacter are predominant components among the indigenous microbial community in siliceous deposits formed in pipes and equipment of Japanese geothermal power plants. These bacteria seem to actively contribute to the rapid formation of huge siliceous deposits. Additionally, in vitro examination suggested that Thermus cells induced the precipitation of supersaturated amorphous silica during the exponential growth phase, concomitant with the production of a specific cell envelope protein (Silica-induced protein, Sip). Sip seems to be a ferric iron-binding periplasmic protein of ABC transporter). The production of Sip was inhibited by Al ion. Dissolved silica in geothermal hot water may be a significant component in the maintenance of position and survival of microorganisms in limited niches.