Fluorescence microscopic evidence for intracellular silicon storage pools in diatoms and their partial separation

Diatoms, unicellular algae found in all types of freshwater and saltwater environments, acquire silicic acid from their surroundings to create species-specific silica-based cell walls, known as frustules. During biosilica formation, silicon is transported from storage pools (SSPs) into a silicon deposition vesicle (SDV). This study focuses on fluorescence microscopic visualization and quantification of silica formed from SSPs during silicon starvation.

Researchers utilized the fluorescence marker PDMPO to demonstrate ongoing cell growth and division without the need for silicon uptake from the environment. In Cyclotella nana, the SSPs may comprise up to one-third of the total cellular silicon content, with intracellular concentrations reaching approximately 540 millimolar. This is significantly higher than the saturation level of 2 millimolar.

The study reports a method for subcellular fractionation of C. nana using density gradient centrifugation. Quantification of cytoplasmic silicon reveals that it accounts for 3 to 10% of the total cellular silicon, corresponding to intracellular concentrations of roughly 50 to 160 millimolar. Labeling with NMR active 29Si indicates that the detected silicon is in the form of condensed silica, as silicic acid is converted into condensed silica after uptake.

However, the extracellular silicon concentrations are lower than the 540 millimolar predicted by fluorescence experiments.

The missing intracellular silicon may be present in the form of large aggregates or compartments. Alternatively, it might also be bound to cell wall biosilica.