Silicification of 'cell walls’ of certain protistan flagellates

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Abstract
Silica deposition is described for two protistan flagellates, Synura petersenii (Chrysophyceae, algae) and Stephanoeca diplocostata (Choanoflagellida, Protozoa). Both taxa produce silica units intracellularly and subsequently assemble them outside the protoplast to form a ‘cell wall’. In Synura the cell wall consists of a scale case to which scales are added throughout the cell cycle. In Stephanoeca individual siliceous, costal strips are accumulated outside the protoplast and assembled into a lorica once sufficient strips have been produced. In both taxa silica is laid down within silica deposition vesicles (s.d.vs) of uncertain origin. Microtubules are involved in the orientation and support of s.d.vs during early stages of silica unit biogenesis. Detailed comparisons of silica deposition are made between Synura and Stephanoeca and between these and other silica-depositing protistans.