The excretion of extracellular polymeric substances (EPS) by an axenic culture of the benthic diatom Cylindrotheca closterium was investigated. Two sequential extraction steps proved to be sufficient to remove the bulk of the EPS present. Soluble EPS was recovered by a simple centrifugation step and represented a fraction that was not or was only loosely associated with diatom cells. For the extraction of bound EPS, different procedures were compared. The best results were obtained using distilled water as extraction solvent (1 h, 30 °C). The sugars that were recovered using this procedure were typically associated with aggregates of diatoms. In addition to the distinct differences in localization of the different types of EPS, their temporal dynamics differed in relation to the light–dark cycle. Soluble EPS were continuously released into the medium at a rate of 1·6 pg cell−1 day−1. In contrast, the production of bound EPS was highly light-dependent. In the dark, this bound EPS rapidly disappeared, probably as the result of its utilization by the 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.