Biosilica Nanofabrication in Diatoms: The Structures and Properties of Regulatory Silaffins
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Biosilica Nanofabrication in Diatoms: The Structures and Properties of Regulatory Silaffins Nils Kröger1,2 and Nicole Poulsen1 1 School of Chemistry and Biochemistry, 2School of Materials Science and Engineering Georgia Institute of Technology, 770 State Street, Atlanta, GA 30332-0400, U.S.A. ABSTRACT Diatoms are a large group of unicellular microalgae encased by silica cell walls that exhibit species-specific, mostly porous micro-and nanopatterns. Previously, from the diatom Cylindrotheca fusiformis unique phosphoproteins (termed silaffins) and unusually long polyamine chains (termed LCPA) have been identified and implicated in silica formation. However, analysis of the general role of silaffins in species-specific silica morphogenesis has been hampered by lack of data about silaffins from other diatom species. Recently, we have isolated the five major silaffins from the diatom Thalassiosira pseudonana and aided by the genome data available from this organisms we were able structurally und functionally characterize these molecules. These data clearly support the hypothesis that silaffins play an important role in the nanofabrication of diatom biosilica. The basic insights into the mechanism of biomineral morphogenesis by silaffins and LCPA suggest future pathways for the fabrication of nanostructured minerals by synthetically available polymers. INTRODUCTION The formation of inorganic materials with complex form (biominerals) is a very widespread biological phenomenon [1]. The most fascinating biomineral structures are found among unicellular organisms, which produce exoskeletons made of silica or calcium carbonate. These structures are intricately patterned at the nano- to micrometer scale and have thus been recognised as paradigms for the controlled production of inorganic nanomaterials [2]. However, the mechanism allowing biomineral morphogenesis to proceed within minutes at extremely mild reaction conditions and with genetically controlled precision is not well understood. To date the molecules involved in biosilica formation have mainly been studied in sponges and diatoms [3]. Diatoms are unicellular algae, and each species is characterized by a specific biosilica cell wall that contains regularly arranged slits or pores typically in the size range between 10 nm to 1000 nm (nanopatterned biosilica, figure 1). Based on biochemical data exclusively obtained from work on the diatom Cylindrotheca fusiformis it has been hypothesized that morphogenesis of diatom biosilica may generally require at least two types of organic molecules: Firstly, components like LCPA [4] or the silaffin peptide natSil1A [5] that accelerate silica formation, and secondly regulatory silaffins like natSil2 [6] that control the nanostructure of the forming silica. To test this hypothesis we have analysed the silica forming components of the diatom Thalassiosira pseudonana, a species that is phylogenetically distant to C. fusiformis and exhibits very different biosilica structures.
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Figure 1. SEM images of diatom biosilica.
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