Silicon: Background Information
Silicon is one of the most abundant elements in the earth's crust, second only to oxygen. Though it is found at such great quantities, most of this silicon is not available to organisms due to its insoluble nature (in rocks, for example). When silicon is in solution it is usually present in the form of orthosilicic acid (Si(OH)4).
Silicon and Diatoms
Since diatoms need silicon in order to build their frustules, they are usually found in regions where there is an abundance of orthosilicic acid. However, though populations of diatoms tend to build rapidly in the presence of silicon, there is usually a sharp decline after the silicon has been depleted. There is a variability amongst diatoms with respect to how much silicon they need to uptake. Thus, those diatoms that are not as dependent on silicate are able to prosper even after their silicon-dependent counterparts die off.
Silicon is taken up at different rates throughout the cell cycle. Thus, you can guess that it is taken up more quickly when the frustule is being created (the new valves and girdle elements). Silicon is acquired by the diatom in an active, carrier-mediated manner across the plasma membrane.
How a Diatom Employs Silicon
The most obvious use of silicon mentioned above is the use of silicon in the siliceous components of the cell (valves, girdle elements). The cell wall is constructed by silica deposition vesicles (SDV) with membranes called silicalemmas. The orthosilicic acid and other forms of silicon in solution are transported to theses SDVs, and polymerization occurs in order to create solid deposits of silica. This polymerization may be sparked by a change in pH or silicon in solution or a host of other factors. The SDV have extensions that become connected, allowing expansion of the cell wall and its associated silicalemma. It should be noted that silica is not laid down in the raphe slits, and it may be prevented from doing so through the presence of raphe fibers.
It may not be as obvious that silicon can be employed in the metabolic processes of diatoms as well. Without silicate, diatom cells cannot divide, and its capacity to perform energy producing processes (photosynthesis and glycolysis) are diminished. In addition, synthesis of DNA, protein, and chlorophyll halts. These effects may be a result of silicate's role in the regulation of some enzymes.
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copyright Jennifer Shin 1999.