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PHYCOLOGICAL SOCIETY OF AMERICA

Pseudo-nitzschia Organic Casing

Description and Function

The organic casing of Pseudo-nitzschia coats the frustule components (the siliceous components) with a very thin layer. It is made up of a coating membrane and diatoteptic layer:

Coating Membrane: The coating membrane is a thin layer that coats the silica on the exterior of the cell. It is thought that this coating helps to protect the frustule while also complexing cations that may help decrease levels of silica dissolution.

Diatoteptic Layer: The diatoteptic layer, unlike the coating membrane, coats the inner layer of the frustule. It is secreted along this inner layer and aids in shape maintenance. Silica embeds itself here, forming a matrix. This layer also helps the cell change its permeability through pore reduction and expansion.

Constitution

Both the coating membrane and the diatoteptic layer of Pseudo-nitzschia 's organic casing are made up of proteins, lipids and sugars. The amino acids found in the casing is distinct from those found in the cytoplasm. More specifically, there are large quantities of threonine, glycine, and serine. It has been hypothesized that these amino acids constitute a protein template that aid in silicification. The hydroxyl groups of these amino acids may create a layer on which silicic acid can gather and polymerize. With respect to lipids, there is a high concentration of oleic acid in the organic casing though its significance is unclear. Sugars are found in the form of beta 1-3 linked glucan, usually located at the edges of the hypocingulum. Their location makes it possible for the casing to help preserve the cell's integrity during cell growth, as the girdle elements slide apart from one another.

Visualization

In order to study the organic casing of Pseudo-nitzschia , it is necessary to obtain the wall components from living diatoms. This may be achieved by cracking frustules with glass beads followed by isolation of the wall components through differential centrifugation. These isolated components then need to be treated with a strong acid in order to dissolve the silica. After the silica is dissolved, the organic casing remains and can then be visualized using electron microscopy.

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copyright Jennifer Shin 1999.