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To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brébisson et Kützing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kützing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3-(2,4-dinitroanilino)-3amino-N-methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion. 1 Key index words: Bacillariophyceae; 2,7-bis-(2-carboxyethyl)-5(and-6)-carboxyfluoresceinacetoxymethylester (BCECF-AM); 3-(2,4-dinitroanilino)-3-amino-N-methyl propylamine (DAMP); frustule; organellar pH; rhodamine 123; silicon deposition vesicle (SDV) Abbreviations: BCECF-AM, 2,7-bis-(2-carboxyethyl)-5(and-6)-carboxyfluoresceinacetoxymethylester; DAMP, 3-(2,4-dinitroanilino)-3-amino-N-methyl propylamine; SDV, silicon deposition vesicle; STV, silicon transport vesicle

The formation and consumption of precursors and the crystallization of Si-MFI using bis(tripropylammonium) hexamethylene dihydroxide (“dimer” of tetrapropylammonium cation, TPA) and bis(tripropylammonium-N-N‘-hexamethylene)-N‘ ‘,N‘ ‘-dipropylammonium trihydroxide (“trimer” of TPA) as structure-directing agents have been investigated in situ using simultaneous, time-resolved, SAXS and WAXS and using USAXS. The formation of 2.8-nm-sized primary units is observed upon dissolution of the silica source, which is in agreement with results from earlier studies on systems with the TPA cation as a structure-directing agent. Aggregation of these nanometer-scale primary units to 10−15-nm-sized particles is found to be an essential step in nucleation of the zeolite. Crystal growth occurs via the addition of the primary units to the growing crystal. Although the size of the primary units for MFI is independent of the structure-directing agent used, the organic species does have a pronounced influence on the crystal growth step and, therefore, on the crystal growth rate, size, and morphology. The results presented here confirm a common mechanism proposed for organic-mediated crystallization of high-silica zeolites.

The formation of precursor particles in the crystallization of zeolites from gelating systems has been studied using X-ray scattering at small angles. The crystallization of Si-MFI using trimethylene-bis(N-hexyl, N-methyl-piperidinium) as the structure-directing agent shows the formation of two categories of precursors: gel particles and nanometer-scaled primary units. The size of the primary units for the crystallization of Si-MFI is found to be 2.8 nm both for gelating and nongelating systems using different structure-directing agents. Zeolites Si-BEA and Si-MTW have been prepared using the same organic (trimethylene-bis(Nbenzyl, N-methyl-piperidinium)) at different concentrations. Primary units with a size of 2.6 nm are found to be present in the synthesis of Si-BEA, while their size is 1.5 nm in the synthesis mixture directing to Si-MTW. Our data suggest that the nanometer-scaled primary units are specific for the zeolite topology formed.

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