Biological processing of nanostructured silica in diatoms

The most outstanding example of biological processing of silicon occurs in the unicellular algae known as diatoms. The diatom cell wall contains nanostructured silica with features exceeding current manufacturing capabilities, reproduced with exactness in vast numbers. Such structures must result from specific molecular interactions between cellular components and silicon, and larger scale movements of the intracellular compartment where silica polymerization occurs, the silica deposition vesicle (SDV). New insights into diatom silicification have arisen from recent characterization of the molecular components involved. We have isolated genes encoding silicic acid transporters (SITs) responsible for transport across the cell's lipid bilayer membrane. The SITs are the only proteins shown to specifically interact with silicon, and a major goal is to identify amino acids responsible for silicic acid recognition and binding. Long-chain polyamines, both free and peptide-attached, apparently induce silica polymerization in diatoms. These compounds have not been shown to have direct control over the formation of larger-scale structure, but observations suggest some involvement. Movements and molding of the SDV during silicification, driven by the cytoskeleton, are major determinants of silica macrostructure. We have applied and are developing techniques to elucidate the molecular mechanisms underlying diatom silicification. This investigation could inspire biomimetic approaches, or lead to the specific manipulation of silicified diatom structures for direct application in nanostructured materials syntheses. These materials are not limited to being silica-based; recent work using shape-preserving displacement reactions has converted diatom silica into an inorganic metal oxide, while maintaining the detailed silica morphology. (C) 2003 Published by Elsevier B.V.