Biomineralization is the process in which various organisms internally or externally produce inorganic minerals as biominerals such as bones, teeth, shells, and invertebrate exoskeletons. The magnetites, iron deposits, gold deposits, calcium carbonates, calcium phosphates, and silicates are well-known examples of the a—biominerals. Although silica is the most abundant compound in the earth’s crust and its precipitation is an important geological process in many geothermal environments, it is not useful for microorganisms. Recent research efforts revealed that both inorganic chemical reactions and microbial activity can be implicated in the formation of siliceous deposits. The extremely thermophilic bacteria within the genus Thermus are predominant component in the indigenous microbial community in siliceous deposits formed in pipes and equipment of geothermal power plants, which contributes to the rapid formation of huge siliceous deposits. In vitro examination suggested that Thermus cells induced precipitation of supersaturated amorphous silica during the exponential growth phase. A silica-induced protein (Sip) was isolated from the cell envelope fraction. The amino acid sequence of Sip was similar to that of the solute-binding protein of the Fe3+-binding ABC transporter. Furthermore, Sip promotes silica deposition on the surfaces of cells, after which the silicified outer membrane may serve as a ‘suit of armor’ that confers resistance to peptide antibiotics. Dissolved silica in geothermal hot water may be a significant component in the maintenance and survival of microorganisms in nutrient-limited niches. And thus, thermophilic bacteria may use biosilicification for their own survival. This chapter reviews the formation of siliceous deposits by thermophilic bacteria in geothermal environments.
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