The Glasshouse of Diatoms

Abstract

Diatoms are unicellular algae which protect their cell body by a highly ornate, silicified cell wall that forms a glasshouse called a frustule. Most diatoms possess a photosynthetic apparatus, and so obviously the cell wall must be transparent. The frustule has many openings to allow for the transport of nutrients and other compounds which are important for metabolism of the diatoms. Silicic acid Si(OH)4 is pumped by membrane spanning transporters into silica deposition vesicles (SDV) in which it exists in very high concentrations. After cell division precursors of a new silicious glass wall are formed by exocytosis from the SDVs. Several proteins were isolated, which may be involved with the process of silica deposition and shape formation. A protein silaffin with repeating domains and unusual modifications at three lysines per repeat was found to catalyse the formation of microspheres in a glass state. It is not clear how the complex shape of the silicious wall is controlled. It is speculated that still unknown proteins acting during exocytosis may be the key players.