9:55 AM - 10:15 AM
[MIS07-04] Intracellular silica biomineralization in bacteria: an overlooked phenomenon in the global Si cycle
★Invited Papers
Keywords:Bacteria, Biomineralization, Silica, Silicon cycle
Silicon (Si), the second most abundant element in the Earth's crust after oxygen, is an important mineral for certain organisms. The oxidized form of silicon, silicon dioxide or silica (SiO2), is the primary building material in the skeletal structures of diatoms, radiolarians, and siliceous sponges. Silica is also utilized by some higher plants (e.g., rice and horsetail) to alleviate biotic and abiotic stresses. In all these organisms, silicon is taken up from the surroundings as soluble silicic acid (Si[OH]4), a biologically available form of silicon in nature. Silicic acid is then polymerized and deposited as insoluble silica through a biological process called biosilicification (silica biomineralization).
All of the biosilicifying organisms mentioned above are eukaryotes, whose cells have a membrane-bound nucleus, and have been the targets of intense research to study the molecular mechanisms underlying biosilicification. By contrast, biosilicification in prokaryotes (i.e., bacteria and archaea), which are more primitive life forms whose cells lack a membrane-bound nucleus, has been less well studied, because the biosilicifying capability of bacteria was not recognized until recently [1]. We previously reported that biosilicification occurs in the gram-positive, spore-forming bacterium Bacillus cereus and its close relatives [2]. These bacteria take up silicic acid from the surroundings into the cell and polymerize it into silica on the surface of their spores to form a capsule-like silica layer (Figure). Spores encapsulated by silica layers showed higher viability under acidic conditions than non-encapsulated spores. These results suggest that the silica layer acts as a protective coating against acid and might contribute to spore survival under acidic conditions. More recently, silicon accumulation in marine cyanobacteria and silica granule formation in magnetotactic bacteria have been reported [3,4], although their physiological roles have not been elucidated.
Silicic acid is present in natural waters, with estimated average concentrations in rivers and oceans of 15 and 7 μg/ml, respectively [5], although concentrations vary significantly between sites. Besides, the concentration of silicic acid in soil solutions normally ranges from 10 to 60 μg/ml [6]. Considering the abundance of silicic acid and the ubiquitous presence of bacteria in nature, bacterial biosilicification activity could be involved in the formation of significant amounts of silica; therefore, although previously unrecognized, this process might make a non-negligible contribution to the global silicon cycle.
All of the biosilicifying organisms mentioned above are eukaryotes, whose cells have a membrane-bound nucleus, and have been the targets of intense research to study the molecular mechanisms underlying biosilicification. By contrast, biosilicification in prokaryotes (i.e., bacteria and archaea), which are more primitive life forms whose cells lack a membrane-bound nucleus, has been less well studied, because the biosilicifying capability of bacteria was not recognized until recently [1]. We previously reported that biosilicification occurs in the gram-positive, spore-forming bacterium Bacillus cereus and its close relatives [2]. These bacteria take up silicic acid from the surroundings into the cell and polymerize it into silica on the surface of their spores to form a capsule-like silica layer (Figure). Spores encapsulated by silica layers showed higher viability under acidic conditions than non-encapsulated spores. These results suggest that the silica layer acts as a protective coating against acid and might contribute to spore survival under acidic conditions. More recently, silicon accumulation in marine cyanobacteria and silica granule formation in magnetotactic bacteria have been reported [3,4], although their physiological roles have not been elucidated.
Silicic acid is present in natural waters, with estimated average concentrations in rivers and oceans of 15 and 7 μg/ml, respectively [5], although concentrations vary significantly between sites. Besides, the concentration of silicic acid in soil solutions normally ranges from 10 to 60 μg/ml [6]. Considering the abundance of silicic acid and the ubiquitous presence of bacteria in nature, bacterial biosilicification activity could be involved in the formation of significant amounts of silica; therefore, although previously unrecognized, this process might make a non-negligible contribution to the global silicon cycle.