Nils Kröger

From Molecular Mechanisms of Biomineralization and Bioadhesion to Applications in Nanobiotechnology

Previous and current research

In the course of millions of years of biological evolution organisms have developed fascinating biomolecule-based capabilities that are unmatched by human technology. Remarkable examples are provided by the diatoms, a large group of eukaryotic microalgae that are responsible for ~20 % of the annual photosynthetic productivity on our planet. Diatoms produce cell walls, which are made of amorphous SiO₂ (glass or silica) and exhibit species specific shapes with intricate nano- and micropatterns. The extraordinary ability of diatoms for silica morphogenesis is a paradigm for the genetically controlled "bottom-up" synthesis of functional inorganic materials with nanopatterned 3D architectures. Previously, we have identified through biochemical analyses unique proteins (silaffins, cingulins; see part A of figure ) and other unusual biomolecules (long-chain polyamines) that are involved in diatom biosilica formation. These studies are greatly aided by bioinformatics analyses of already completed and forthcoming diatom genome projects, as well as the development of genetic engineering tools for diatoms. We aim to structurally and functionally characterize the entire biomolecular machinery for silica biogenesis. Furthermore, we have already begun harnessing the emerging insight into this process to develop novel, envronmentally benign methods (see parts B and C in figure) for the synthesis of organic-inorganic hybrid materials with advanced functionalities for a wide variety of applications including bioenergy and biomedicine. Many diatoms have the amazing capabilities to reversibly adhere to vitrually any surface (hydrophilic or hydrophobic) under water. The molecules and mechanism that control this adhesion process are unknown, yet they could be highly useful glues for technology and in biomedicine. We aim to identify the chemical structure of diatom adhesives, elucidate the mechanism of diatom adhesion, and will attempt to mimic this process by synthetic analogs of diatom adhesion molecules.

Fig:  (A) Schematic chemical structure of a silica forming peptide from diatoms. The peptide carries unique posttranslational modifications that are essential for the silica forming activity. (B) Top: SEM image of organic microrings isolated from diatom biosilica that exhibit characteristic filament nanopatterns. Bottom: Confocal fluorescence microscopy images of three transgenic diatom cells that incorporate GFP-fusion proteins into specific regions of the biosilica. (C) Schematic of peptide-mediated Layer-by-Layer mineralization allowing for the stepwise surface deposition of conformal and continuous mineral layers with nanoscopic thickness. (D) Light microscopy image (after staining with the dye "Stains All") of an individual diatom cell (asterisk) attached to a submersed hydrophobic surface. The cell has moved across the surface leaving behind adhesive trails.

Future projects and goals

- Molecular composition and self-assembly of silica forming organic templates in diatoms

- Intracellular targeting of proteins to the silica deposition vesicles in diatoms

- Pathway for biosynthesis of long-chain polyamines in diatoms 

- Structural and functional characterization of underwater adhesives from diatoms

- Diatom Nanobiotechnology: Genetically engineered hybrid organic-inorganic materials with hierarchical 3D nano- and micropatterns and designed functonalities

- Biomimetic and bioenabled syntheses of (bio)catalytic materials for bioenergy and biomedical applications

About

1986-2091: Chemistry Studies at Universities of Marburg and Regensburg (Germany) 1995: Dr. rer. nat. (PhD) in Biochemistry at University of Regensburg 1996-1997: Postdoctoral Fellow, Department of Biochemistry, Genetics and Microbiology at University Regensburg   1998: DFG Research Fellow, School of Botany (Biological Electron Microscopy) at University Melbourne (Australia) 1998-2004: Group Leader, Department of Biochemistry, Genetics and Microbiology at University Regensburg 2001: Habilitation in Biochemistry, University Regensburg 2005-2011: Assistant Professor, School of Chemistry & Biochemistry and School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta (USA) 2011: Associate Professor with tenure 2012: Full Professor, B CUBE Center and Department of Chemistry, Dresden University of Technology (Germany)

Selected publications

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.

V. C. Sheppard, N. Poulsen, N. Kröger (2010): Characterization of an endoplasmic reticulum-associated silaffin kinase from the diatom Thalassiosira pseudonana. J. Biol. Chem. 285, 1166-1176.

Y. Fang, Q. Wu, M. B. Dickerson, Y. Cai, S. Shian, J. Berrigan, N. Poulsen, N. Kröger, K. H. Sandhage (2009): Protein-Mediated Layer-by-Layer Syntheses of Freestanding Microscale Titania Structures with Biologically Assembled 3-D Morphologies. Chem. Mater. 21, 5704-5710.

N. Poulsen, C. Berne, J. Spain, N. Kröger, (2007): Silica immobilization of an enzyme via genetic engineering of the diatom Thalassiosira pseudonana. Angew. Chem. Int. Ed. 46, 1843-1846.

N. Kröger, S. Lorenz, E. Brunner, M. Sumper (2002): Self-assembly of highly phosphorylated silaffins and their function in biosilica morphogenesis. Science 298, 584-586.

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