Salinity-dependent diatom biosilicification implies an important role of external ionic strength

Abstract

The role of external ionic strength in diatom biosilica formation was assessed by monitoring the nanostructural changes in the biosilica of the two marine diatom species Thalassiosira punctigera and Thalassiosira weissflogii that was obtained from cultures grown at two distinct salinities. Using physicochemical methods, we found that at lower salinity the specific surface area, the fractal dimensions, and the size of mesopores present in the biosilica decreased. Diatom biosilica appears to be denser at the lower salinity that was applied. This phenomenon can be explained by assuming aggregation of smaller coalescing silica particles inside the silica deposition vesicle, which would be in line with principles in silica chemistry. Apparently, external ionic strength has an important effect on diatom biosilica formation, making it tempting to propose that uptake of silicic acid and other external ions may take place simultaneously. Uptake and transport of reactants in the proximity of the expanding silica deposition vesicle, by (macro)pinocytosis, are more likely than intracellular stabilization and transport of silica precursors at the high concentrations that are necessary for the formation of the siliceous frustule components.biosilica ͉ silica nanostructure ͉ silicification ͉ silica chemistry D iatoms are known for the intriguing species-specific morphology of their siliceous exoskeletons, the frustules (1, 2), which consist of two silica valves (the epi-and hypovalve), siliceous girdle bands, and a protective organic casing that prevents dissolution of the siliceous parts in the aquatic environment. During cell division, a parental diatom cell undergoes cytokinesis, and each of the next two daughter cells produces a new hypovalve, a hypocingulum, and girdle band(s); the sequence of their formation differs among species (1, 2). After valve and girdle band completion, the daughter cells finally separate, and cell division continues. The silica of every new valve is formed in the silica deposition vesicle (SDV) that is located inside each daughter cell and is closely appressed to the plasma membrane along the cleavage furrow. The SDV rapidly expands two-dimensionally and subsequently thickens more slowly in the three-dimensional direction during formation of the new hypovalve (3, 4). In the course of this fast two-dimensional expansion process, the essential reactants for silica polymerization have to be transported efficiently to the SDV. Silicon transporters have been identified (5, 6), but there is no clear evidence that they transport sufficient amounts of silicic acid across the plasma membrane and/or SDV membrane to enable silica polymerization.In diatoms, both organic and inorganic compounds foster silica biomineralization and possibly control silica precipitation and direct the structures formed (7-9). Relevant organic compounds are silica-precipitating peptides such as silaffins and long-chain polyamines (8-10). They all have an intracellular origin and require specific targeting or intracellular transpo...

Bio-manufacturing technology based on diatom micro- and nanostructure

“…The structure changes greatly during its formation, and thus different structures of the valve could be obtained by acid-washing the cells at different stages. Moreover, the formation of frustule can be affected by the mother liquor in SDV, such as H + [53], salt [54] and metal ions [55,56]. In another study, Jeffryes et al [56,57] added 50 μmol/L of Ge(OH) 4 to the culture medium, which reduced the pore size of the newly generated frustules from 203 to 124 nm, and a frustule framework structure was obtained.…”

Compartmental Analysis Suggests Macropinocytosis at the Onset of Diatom Valve Formation

“…A decreasing pH in the SDV upon diatom valve maturation suggests enhancement of the finalization of the silica polymerization reaction [6]. Ionic strength determines the degree of condensation of the silica and affects the nanostructure of the solid silica formed in diatoms [7]. Noteworthy is that in diatoms early valve formation coincides with nanostructuring of the primer two-dimensional silica base [16,17].…”

“…To explain a direct influence an alternative silicon uptake pathway by (macro)pinocytosis has been proposed [7]. It remains to be shown whether or not this process takes place by: 1) 'classical' pinocytosis in which multiple smaller vesicles are involved, or 2) macropinocytosis in which one larger vesicle is formed which transforms to become the initial SDV.…”

“…Noteworthy is that in diatoms early valve formation coincides with nanostructuring of the primer two-dimensional silica base [16,17]. The impact of reaction conditions on silica polymerization in diatom frustule formation suggests that cellular transport routes during which polymerization of accumulated silicon species could occur are not likely to be involved in valve formation [4,7]. Silicon in diatom cells (excluding the silica of the frustule) has been proposed to be present in a precondensed silica sol-like form [18], while during valve formation organo-silicon interactions occur [19].…”

“…The silicon required for the formation of the frustule has to be taken up by the cell and transported to the cellular compartment in which the siliceous parts of the frustule are formed: the SDV [1]. Silica formation in diatoms integrates silica chemistry, silicic acid polymerization and condensation [4,6,7] on the one hand and molecular pathways on the other [8,9]. Much attention has so far been paid to the uptake of silicon by cells and Electronic supplementary material The online version of this article (doi:10.1007/s12633-010-9059-2) contains supplementary material, which is available to authorized users.…”