We studied the growth of the araphid pennate diatom Synedra acus subsp. radians (Kützing) Skabichevskii using a
fluorescent dye N(1),N(3)-dimethyl-N(1)-(7-nitro-2,1,3-benzoxadiazol-4-yl)propane-1,3-diamine (NBD-N2), which stains growing siliceous frustules but does not
stain other subcellular organelles. We used a clonal culture of S. acus that was synchronized by
silicon starvation. Epifluorescence microscopy was performed in two different ways with cells stained by the addition of
silicic acid and the
dye. Individual cells immobilized on glass were observed during the first 15-20 min following the replenishment of
silicic acid after
silicon starvation. Alternatively, we examined cells of a batch culture at time intervals during 36 h after the replenishment of
silicic acid using fluorescence and confocal microscopy. The addition of
silicic acid and NBD-N2 resulted in the rapid (1-2 min) formation of several dozen green fluorescent submicrometer particles (GFSPs) in the cytoplasm, which was accompanied by the accumulation of fluorescent
silica inside
silica deposition vesicles (SDVs) along their full length. In 5-15 min, GFSPs disappeared from the cytoplasm. Mature siliceous valves were formed within the SDVs during the subsequent 14-16 h. In the next 8-10 h, GFSPs appeared again in the cytoplasm of daughter cells. The data obtained confirm observations about the two-stage mechanism of
silicon assimilation, which includes rapid
silicon uptake (surge uptake) followed by slow
silica deposition. It is likely that the observed GFSPs are
silicon transport vesicles, which were first proposed by Schmid and Schulz in (Protoplasma 100:267-288, 1979).