No abstract

Silicon is involved in numerous important structural and functional roles in a wide range of organisms, including diatoms, plants, and humans, but clear mechanisms have been discovered only in diatoms and sponges. Silicate availability influences metal concentrations within various cell-and tissue-types, but a mechanism has not been discovered so far. In an earlier study on Baker's yeast Saccharomyces cerevisiae it was proposed that a chemical mechanism, rather than a biological one, is important. In the present study, the interaction of silicon with Baker's yeast is further investigated by studying the influence of zinc and magnesium on Si accumulation both at a low and a high silicate concentration in the medium. Si accumulation fitted well with Freundlich adsorption and Si release followed depolymerization kinetics, indicating that silicate adsorbs to the surface of the cell rather than being transported over the cell membrane. Subsequently, adsorbed silicate interacts with metal ions and, therefore, alters the cell's affinity for these ions. Since several metals are nutritional, these Si interactions can significantly change the growth and viability of organisms. In conclusion, the results show that chemistry is important in Si and metal accumulation in Baker's yeast, and suggest that similar mechanisms should be studied in detail in other organisms to unravel essential roles of Si.

Silicon (Si, as silicate) is involved in numerous important structure and function roles in a wide range of organisms, including man. Silicate availability influences metal concentrations within various cell and tissue types, but, as yet, clear mechanisms for such an influence have been discovered only within the diatoms and sponges. In this study, the influence of silicate on the intracellular accumulation of metals was investigated in baker's yeast (Saccharomyces cerevisiae). It was found that at concentrations up to 10 mM, silicate did not influence the growth rate of S. cerevisiae within a standard complete medium. However, an 11% growth inhibition was observed when silicate was present at 100 mM. Intracellular metal concentrations were investigated in yeast cultures grown without added silicate (-Si) or with the addition of 10 mM silicate (+Si). Decreased amounts of Co (52%), Mn (35%), and Fe (20%) were found within +Sigrown yeast cultures as compared to -Si-grown ones, whereas increased amounts of Mo (56%) and Mg (38%) were found. The amounts of Zn and K were apparently unaffected by the presence of silicon. +Si enhanced the yeast growth rate for low-Zn2+ medium, but it decreased the growth rate under conditions of a low Mg2+ medium and did not alter the growth rates in high Zn2+ and Co2+ media. +Si doubled the uptake rate of Co2+ but did not influence that of Zn2+. We propose that a possible explanation for these results is that polysilicate formation at the cell wall changes the cell wall binding capacity for metal ions. The toxicity of silicate was compared to germanium (Ge, as GeO2), a member of the same group of elements as Si (group 14). Hence, Si and Ge are chemically similar, but silicate starts to polymerize to oligomers above 5 mM, whereas Ge salts remain as monomers at such concentrations. Ge proved to be far more toxic to yeast than Si and no influence of Si on Ge toxicity was found. We propose that these results relate to differences in cellular uptake.

During valve formation of the siliceous frustules of diatoms, bulk uptake of silicic acid and its subsequent transport through the cell is required before it can be deposited in the silica deposition vesicle (SDV). It has been assumed that transport takes place via silicon transporters (SITs), but if that were the case a control mechanism would have to exist for stabilization of the large amounts of reactive silicon species during their passage through the cell on the way to the SDV. There is, however, no reason to assume that classical silica chemistry does not apply at elevated levels of silicic acid, and therefore autopolymerization could reasonably be expected to occur. In order to find alternative ways of Si transport that correspond with the high speed of valve formation at the earliest stages of cell division we followed 31 Si(OH) 4 uptake in synchronously dividing cells of the diatoms Coscinodiscus wailesii, Navicula pelliculosa, N. salinarum, and Pleurosira laevis.The results were related to systematically derived mathematical models for a compartmental analysis of 5 possible uptake/transport pathways, including one involving SITs and one involving (macro)pinocytosis-mediated uptake from the extracellular environment. Our study indicates that the uptake of radioactive silicic acid matches best with the model that describes macropinocytosis-mediated silicon uptake. This process is well in line with the observed 'surge uptake' at the start of valve formation when the demand for silicon is high; it infers that in diatoms a pathway of uptake and transport exists in which SITs are not involved.

SummaryNo-carrier-added 31 Si was produced by 31 P(n,p) 31 Si in the nuclear reactor of the Delft University of Technology. Several methods were investigated to remove the side product 32 P, and the chemical form of 31 Si was investigated. All methods except one gave good results. Anion exchange with Dowex resin gave the best results in activity concentration (3.8 Â 10 6 Bq/ml, with only 3.2 Â 10 3 Bq/ml 32 P as by-product) and specific activity (A s >17 TBq/g). The product is suitable for biological systems.