Growth and silica content of the diatoms Thalassiosira weissflogii and Navicula salinarum at different salinities and enrichments with aluminium

The dependence of the cellular (biogenic) and frustule-associated (minerali zed) silica content of the diatoms Navicula salinarum and Thalassiosira wei ssflogii on salinity and aluminium conditions was studied in order to make it possible to manipulate silicification in vitro and maximize it to levels required for physico-chemical frustule characterization by physisorption, X-ray scattering analysis and NMR. which is our ultimate objective. Enrichm ents with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T, weissflogii. Aluminium addi tions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silic a matrix of the frustule. In contrast, lowering the salinity from 28 practi cal salinity units (PSU) to 20 and 15 resulted in a significant increase in the biogenic silica content per cell of both species, which is in line wit h an increase in density of the chemically derived silica under low salt co nditions. The silica content per cell was variable during culture growth; i ncrease in cell densities (during exponential growth) was accompanied by a decrease in contents of biogenic silica per cell. Electron microscopy and e nergy dispersive X-ray analysis (EDXA) support the chemical analyses and al so suggest higher biogenic silica contents at lower salinities. The results indicate that besides silicate concentration and pH, the concentration of salts is an important inorganic factor that affects the silica polymerizati on inside the silica deposition vesicle of diatoms.