 |
Nils KrögerAssistant Professor Office: MSE 2100P Phone: 404-894-4228 Fax: 404-894-7452 |
B.Sc., Philipps University of Marburg (Germany); M.Sc., University of Regensburg (Germany); Ph.D., University of Regensburg
Blanchard Assistant Professorship, 2007; Cottrell Scholar Award, 2008; NSF CAREER Award, 2009Research Interests
Biomineralization
The formation of inorganic materials with complex form (biomineralization) is a widespread biological phenomenon that occurs in almost all groups of organisms from prokaryotes (e.g., magnetite nanocrystals in certain bacteria) to humans (bone and teeth). Among the most spectacular examples of biomineralization are the intricately structured cell walls of diatoms, a large group of single-celled eukaryotic algae that are present in almost all water habitats. Diatom cell walls are made of amorphous, hydrated SiO2 (silica) and exhibit highly regular porous patterns which are hierarchically arranged from the nano- to micrometer scale. The silica structures are produced by polycondensation of Si(OH)4 (silicic acid) molecules which occurs in a specific intracellular compartment, termed the silica deposition vesicle (SDV). These complex biomineral structures reveal our limited understanding of a fundamental biological question: How does a cell translate linear DNA sequence information into patterned three-dimensional structures? Therefore, important lessons will be learned from diatoms regarding the mechanism by which eukaryotic cells assemble their cellular machinery to execute a genetically encoded morphogenetic program. Furthermore, owing to the structural intricacies and exceptional mechanical properties of biominerals, and their formation at mild (i.e., physiological) conditions, biomineralization is regarded as a paradigm for the development of novel routes for the synthesis of functional materials with nanometer precision in three dimensions (Bio-Nanotechnology).
A central aim of my group is to understand the mechanism of silica biomineralization in diatoms by identifying and characterizing the biomolecules involved in this process, investigating their self-assembly properties, and analyzing their silica formation properties in vitro. This biochemical approach builds on previous research that has led to the identification of the first biomolecules involved in diatom silica formation, termed silaffins, silacidins and long-chain polyamines. In collaborative efforts the Kröger group is involved in diatom genome projects, and utilizes bioinformatics approaches to identify new biomineralization proteins in diatom genomes.
Bio-Nanotechnology
A remarkable characteristic of biomineralization is the precise control of complex mineral structures over several orders of magnitude in length scale (from a few nanometers up to millimeters or more) and over several dimensions. This is achieved by the action of highly organized assemblies of cellular macromolecules that enable mineral formation to proceed under mild (i.e., physiological) reaction conditions. The Kröger group aims to translate insights from the molecular mechanisms of biomineralization into novel routes for synthesizing functional inorganic materials with controlled architectures on the nanoscale for various applications (e.g., catalysis, photonics).
In collaborative efforts my group has developed recombinant proteins and peptides that can be applied for the syntheses of silica and titania materials from aqueous solution at near neutral pH and room temperature. Remarkably, the recombinant silaffin rSilC, an E. coli expressed derivative of a natural silaffin from the diatom Cylindrotheca fusiformis, is able to induce the formation of unique rutile TiO2 structures from an aqueous precursor solution . The application of silica and titania forming biomolecules for the controlled deposition of functional mineral coatings in combination with soft materials that are temperature- and/or pH sensitive is currently being explored.
The silica structures produced by diatoms are low-cost materials that exhibit many features interesting for a broad range of nanotechnological applications including membranes for biomolecule separations, size-selective sensors, microelectro-mechanical systems (MEMS), and masks for microfabrication. To fully exploit the nanotechnological potential of diatom silica, my group has developed a novel method to introduce additional functionalities into the diatom silica. This functionalization method is based on molecular genetic engineering of the silica biomineralization process in the diatom T. pseudonana. Recombinant genes that encode a silaffin fused to a functional protein of choice, have been incorporated into the T. pseudonana genome. The silaffin-domain is responsible for targeting the fusion protein for incorporation into the forming silica, thus stably immobilizing the functional domain in the frustule after completion of silica biogenesis. The method has been successfully used to immobilize in the diatom silica the green fluorescent protein (GFP) and various enzymes. Attachment to the biosilica had a stabilizing effect on the protein. Because of the excellent mechanical and mass transport properties of diatom silica enzyme-functionalized diatom cell walls are attractive materials for flow-through devices from ultra small (microfluidics) to large scale (flow-through reactors).
Biofuels
Single-celled microalgae are the most efficient producers of biomass from CO2 and sunlight by far exceeding the production rate per area of terrestrial plants. Biodiesel and bioethanol from microalgae-produced lipids and polysaccharides, respectively, are under investigation as CO2-neutral renewable energy sources for replacing fossil fuels. Marine diatoms are among the most prolific producers of photosynthetic biomass in the ocean. However, the biochemical mechanisms that control the biosynthesis and accumulation of lipids and polysaccharides in diatoms and other algae are largely unstudied. The aim of group is to utilize the information from diatom genome projects and functional genomics studies on the carbon cycle in diatoms to generate metabolically engineered diatom strains with increased production of energy-relevant biomolecules. This will be approached by applying established methods for the genetic transformation of diatoms as well as developing new molecular genetic tools. In collaborative efforts with we are also investigating the process design principles and economics for a plant that would allow the large scale production of biofuels from diatoms.
Selected Publications
Sheppard, V., Poulsen, N. Kröger, N. (2009) Biogenesis of biomineral forming phosphoproteins: Characterization of an endoplasmic reticulum-associated silaffin kinase from the diatom Thalassiosira pseudonana. J. Biol. Chem. in press.
Kröger, N. (2009). The molecular basis of nacre formation. Science 325, 1351-1352.
Bowler, C. et al. (2008). The Phaeodactylum genome reveals the evolutionary history of diatom genomes. Nature. 456, 239-244.
Kharlampieva, E., Tsukruk, T., Slocik, J.M., Ko, H., Poulsen, N., Naik, R.R., Kröger, N., Tsukruk, V.V. (2008) Bio-enabled Surface-mediated Growth of Uniform Titania Nanoparticles. Adv. Mater. 20, 3274-3279.
Kröger, N. and Poulsen, N. (2008) Diatoms- from cell wall biogenesis to nanotechnology. Annu. Rev. Genet. 42, 83-107.
Poulsen, N., Berne, C., Spain, J. and Kröger, N. (2007) Silica immobilization of an enzyme via genetic engineering of the diatom Thalassiosira pseudonana. Angew. Chem. Int. Ed. 46, 1843-1846.
Ahmad, G., Dickerson, M. B., Church, C., Cai, Y., Jones, S. E., Naik, R. R., King, J. S., Summers, J. C., Kröger, N. and Sandhage, K. H. (2006) Rapid, Room-Temperature Formation of Crystalline Calcium Molybdate Phosphor Microparticles via Peptide-Induced Precipitation. Adv. Mater. 18, 1759-1763.
Kröger, N., Dickerson, M.B., Ahmad, G., Cai, Y., Haluska, M.S., Sandhage, K.H., Poulsen, N. and Sheppard, V.C. (2006) Bio-enabled synthesis of Rutile (TiO2) at ambient temperature and neutral pH. Angew. Chem. Int. Ed. 45, 7239-7243.
Ambrust, E. V. et al. (2004). The genome of the diatom Thalassiosira pseudonana: ecology, evolution, and metabolism. Science 306, 79-86.
Poulsen, N., and Kröger, N. (2004) Silica morphogenesis by alternative processing of silaffins in the diatom Thalassiosira pseudonana. J. Biol. Chem. 279, 42993-42999.
Poulsen, N., Sumper, M. and Kröger, N. (2003) Biosilica formation in diatoms: Characterization of native silaffin-2 and its role in silica morphogenesis. Proc. Natl. Acad. Sci. USA 100, 12075-12080.
Kröger, N., Lorenz, S., Brunner, E. and Sumper, M. (2002) Self-assembly of highly phosphorylated silaffins and their function in biosilica morphogenesis. Science 298, 584-586.
Kröger, N., Deutzmann, R., Bergsdorf, C. and Sumper, M. (2000) Species specific polyamines from diatoms control silica morphology. Proc. Natl. Acad. Sci. USA 97, 14133-14138.
Kröger, N., Deutzmann, R., and Sumper, M. (1999) Polycationic peptides from diatom biosilica that direct silica nanosphere formation. Science 286, 1129-1132.