The dependence of the cellular (biogenic) and frustule-associated (mineralized) silica content of the diatoms Navicula salinarum and Thalassiosira weissflogii 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. Enrichments with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T. weissflogii. Aluminium additions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silica matrix of the frustule. In contrast, lowering the salinity from 28 practical 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 with an increase in density of the chemically derived silica under low salt conditions. The silica content per cell was variable during culture growth; increase in cell densities (during exponential growth) was accompanied by a decrease in contents of biogenic silica per cell. Electron microscopy and energy dispersive X-ray analysis (EDXA) support the chemical analyses and also 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 polymerization inside the silica deposition vesicle of diatoms.

The production and composition of extracellular polymeric substances (EPS) in axenic batch cultures of the benthic marine epipelic diatoms Navicula salinarum and Cylindrotheca closterium were investigated. EPS was secreted into the medium and the bulk was loosely associated with the cells. Neither N. salinarum nor C. closterium formed a well-defined polysaccharide capsule. EPS of both N. salinarum and C. closterium consisted predominantly of polysaccharide but small quantities of protein were present as well. EPS also contained uronic acids and SO42− groups. Analysis of monosaccharides using gas chromatography showed that for both species glucose and xylose were the main constituents, but several other monosaccharides were present in smaller quantities. Two fractions of EPS were distinguished: a small amount was secreted into the medium and a second fraction was extracted in water at 30°C. For both species the two fractions differed somewhat in composition, indicating that they represented two different types of EPS. The EPS produced by N. salinarum and by C. closterium differed in their composition. The rate of EPS production in batch culture was highest during the transition from exponential growth to stationary growth. Negatively charged groups such as uronic acids and sulphated sugars determine the adhesion capacity of EPS and probably play an important role in the stabilization of intertidal sediments on which these diatoms grow and produce biofilms.