The formation of inorganic materials with complex form is a widespread biological phenomenon (biomineralization) 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).
Scanning electron microscopy images of cell walls from different diatom species. Images in top and middle row show overviews and the bottom row shows details of individual cell walls.
The aim of the Kröger 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 collaboration with the groups of Ginger Armbrust (University of Washington, Seattle) and Thomas Mock (University of East Anglia, U.K.) the Kröger group is involved in diatom genome projects, and utilizes bioinformatics approaches to identify new biomineralization proteins in diatom genomes.
To investigate the location and possible function of putative biomineralization proteins in vivo, the Kröger group has developed a genetic transformation system for Thalassiosira pseudonana, the model diatom for silica biomineralization studies. This allows for expression of GFP fusion proteins in vivo where we can follow their location in the cell during different stages of the cell cycle and silica morphogenesis.
Expression of a GFP-tagged silaffin tpSil3 in T. pseudonana. The top left image shows a scanning electron micrograph of T. pseudonana. The micrographs on the right show light microscopy images (brightfield, and confocal fluorescence microscopy) of T. pseudonana cells expressing a silaffin-GFP fusion protein. The red fluorescence is caused by the chloroplasts. In interphase cells the silaffin fusion protein is located in all parts of the cell wall (top). During cell division the silaffin-GFP fusion protein becomes incorporated into the newly forming valve part of the silica (bottom).