A role for the cell-wall protein silacidin in cell size of the diatom Thalassiosira pseudonana.

Diatoms contribute 20% of global primary production and form the basis of many marine food webs. Although their species diversity correlates with broad diversity in cell size, there is also an intraspecific cell-size plasticity owing to sexual reproduction and varying environmental conditions. However, despite the ecological significance of the diatom cell size for food-web structure and global biogeochemical cycles, our knowledge about genes underpinning the size of diatom cells remains elusive. Here, a combination of reverse genetics, experimental evolution and comparative RNA-sequencing analyses enabled us to identify a previously unknown genetic control of cell size in the diatom Thalassiosira pseudonana. In particular, the targeted deregulation of the expression of the cell-wall protein silacidin caused a significant increase in valve diameter. Remarkably, the natural downregulation of the silacidin gene transcript due to experimental evolution under low temperature also correlated with cell-size increase. Our data give first evidence for a genetically controlled regulation of cell size in T. pseudonana and possibly other centric diatoms as they also encode the silacidin gene in their genomes.

Analytical studies on the incorporation of aluminium in the cell walls of the marine diatom Stephanopyxis turris.

The eukaryotic diatoms are unicellular algae. They are well known for their filigree micro- and nanostructured cell walls which mainly consist of amorphous silica as well as various organic compounds. However, diatoms are also known to incorporate certain amounts of aluminium into their cell walls. Unexpectedly, enhanced Al concentrations in the Southern Yellow Sea were found to be correlated with a diatom spring bloom. Therefore, we have analyzed the influence of strongly enhanced Al concentrations in the culture medium upon the growth behaviour of the diatom Stephanopyxis turris (S. turris). The uptake and incorporation of Al into the cell walls was monitored. It turned out that S. turris survives aluminium concentrations up to 105.5 µM (2.85 mg/l) in the culture medium. Under the applied conditions, this corresponds to an Al/Si ratio of 1:1. These large amounts of Al had to be offered in the form of bis-tris-chelates in order to prevent uncontrolled precipitation. Under these conditions, the Al/Si ratio in the cell walls could be increased up to about 1:15 as determined by ICP-OES, the highest amount of aluminium found in diatom cell walls yet. Structural characterization of the biosilica by ATR-FTIR and solid-state (27)Al NMR spectroscopy revealed that an amorphous aluminosilicate phase is formed where the aluminium exists as four- and sixfold-coordinated species.

Biomineralization in diatoms-phosphorylated saccharides are part of Stephanopyxis turris biosilica.

Diatoms-unicellular algae with silicified cell walls-have become model organisms for investigations of biomineralization processes. Numerous studies suggest the importance of biosilica-associated or even embedded biomolecules for the biosilica formation. Such molecules are peptides, polyamines, and even saccharides. However, the role of the latter class of biomolecules is only poorly understood yet. Therefore, we investigated the saccharide composition of the biosilica-associated organic material of the diatom Stephanopyxis turris. This species exhibits a considerably high saccharide content in its siliceous cell walls. Gas chromatography-mass spectrometry analysis revealed that mannose-6-phosphate is strongly associated to the cell walls. This phosphorylated saccharide has not yet been found in diatom biosilica. In vitro studies on the polyallylamine-induced silica precipitation were carried out in the presence of mannose-6-phosphate. Compared to inorganic phosphate, mannose-6-phosphate significantly influenced the precipitation behavior of this model system suggesting a possible contribution of mannose-6-phosphate to the biomineralization process of Stephanopyxis turris.

Spatially resolved determination of the structure and composition of diatom cell walls by Raman and FTIR imaging.

Vibrational spectroscopic imaging has developed into a versatile tool to study the local composition of various materials. Here, we present for the first time that Raman mapping and Fourier transform infrared imaging are useful tools to study diatom cell walls as is demonstrated for the species Stephanopyxis turris. The unicellular diatoms exhibit intricately micro- and nano-patterned cell walls, which consist of amorphous silica as well as various organic and inorganic constituents, thus making up an extremely interesting inorganic/organic hybrid material. The structure and composition of this material as well as the biochemical and biophysical processes leading to its formation remain to be challenges for ongoing research. Whereas the lateral resolution of Fourier transform infrared imaging is limited to 5 microm by diffraction, Raman maps are shown to be capable of detecting the spatial distribution of the silica as well as an additional inorganic component and the organic material down to 330-nm resolution. Due to the spherical shape of the sample with a radius of 40 microm and the requirement to accurately focus the laser before each Raman measurement within the micrometer range, Raman maps of whole diatom cell walls were registered after an adjustment of the axial position. The results reveal local differences in the cell wall composition of the honeycomb-like structures and the bottom layer.