Diatoms are unicellular eukaryotic microalgae that produce intricately structured cell walls made of amorphous hydrated SiO2 (silica; Figure 1).  They are arguably the most species-rich group of algae (15), have conquered most aquatic habitats, and play a major role in the global ecosystem, particularly the carbon and silicon cycles The formation of diatom silica is a particularly fascinating example of the biological formation of minerals (biomineralization).

Figure 1. Scanning electron microscopy (SEM) images of the cell walls of four different diatom species.

In previous research the structures and properties of hydrophilic (?soluble?) diatom silica-associated biomolecules (long-chain polyamines and proteins termed silaffins and silacidins) have been extensively investigated. These pioneering studies have provided the first insights into the molecular mechanisms of biological silica formation [1, 2].  However, they identified only a small subset of the silica forming cellular machinery leaving an insurmountable gap in mechanistic understanding between the properties of the currently known silica forming biomolecules and the process of silica morphogenesis in vivo. Recently, diatom silica-associated organic matrices have been identified, that exhibit characteristic nanopatterns, and are insoluble in aqueous solution [2, 3]. Nanopatterned insoluble organic matrices are also present in other biominerals (e.g., sponge silica, mollusk calcite) suggesting a fundamental role in biological mineral morphogenesis. The groups of the Research Unit FOR will analyze the biomolecular composition and assembly of the insoluble organic matrices, study their interaction with the soluble silica-associated biomolecules, and characterize in detail the structure of the bioorganic-inorganic interface.

Like many other biomineralization processes biological silica formation takes place in intracellular lipid bilayer-bound compartments, called silica deposition vesicles (SDVs). To date, the role of lipid bilayer-bound compartments in biomolecule-controlled silica morphogenesis has remained largely unexplored. We address this question by investigating the influence of lipid bilayer-bound microcompartments on the self-assembly of the silica forming biomolecules, and on silica morphogenesis. Furthermore, we attempt isolation of SDVs to characterize their biochemical composition and properties. By in vitro reconstitution of the silica forming machinery from synthetic molecules in combination with computational modeling we aim to obtain a detailed picture of the silica morphogenesis process from the sub-nanometer to the micrometer scale.

The already existing molecular data on diatom silica biomineralization, the plethora of available diatom genome sequence information, and the available tools for genetic manipulation of diatoms make this group of organisms the best model system for studying the molecular fundamentals of biological silica mineralization. We anticipate that the work of the Research Unit will not only elucidate generic molecular principles of biomineral morphogenesis, but also be relevant for other research fields including biomolecular self-organization, organic-inorganic hybrid materials synthesis, and nanomaterials science [1, 5, 6].

[1] N. Kröger, N. Poulsen (2008) Diatoms-From Cell Wall Biogenesis to Nanotechnology. Annu. Rev. Genet. 42, 83-107.

[2] M. Sumper, E. Brunner (2008) Silica biomineralisation in diatoms: The model organism Thalassiosira pseudonana. ChemBioChem 9, 1187-1194.

[3] E. Brunner, P. Richthammer, H. Ehrlich, S. Paasch, P. Simon, S. Ueberlein, K.-H. van Pée (2009) Chitin-based organic networks: An integral part of cell wall biosilica in the diatom Thalassiosira pseudonana. Angew. Chem. Int. Ed. 48, 9724-9727.

[4] A. Scheffel, N. Poulsen, S. Shian, N. Kröger (2011) Nanopatterned protein microrings from a diatom that direct silica morphogenesis. Proc. Natl. Acad. Sci. USA 108, 3175-3180.

[5] M. Sumper, E. Brunner (2006) Learning from diatoms: nature?s tools for the production of nanostructured silica. Adv. Funct. Mater.16,17?26

[6] N. Kröger, E. Brunner (2014) Complex-shaped microbial biominerals for nanotechnology. WIREs Nanomed. Nanobiotechnol. 6, 615?627.