The complex polysaccharides of the raphid diatom Pinnularia viridis (Bacillariophyceae)

A combination of carbohydrate analysis and atomic force microscopy (AFM) was used to characterize the polysaccharides of the pennate diatom, Pinnularia viridis (Nitzsch) Ehrenberg. Polymeric substances were fractionated into those in the spent culture medium (SCM) and those sequentially extracted from the cells with water at 45degreesC (WW), NaHCO3 containing EDTA at 95degrees C (HB), and 1 M NaOH containing NaBH4 at 95degreesC. Carbohydrate, protein, and sulfate were detected in all the fractions, but their relative proportions differed significantly. Nineteen sugars were identified, including pentoses, hexoses, 6-deoxyhexoses, O-methylated sugars, aminohexoses, and traces of uronic acids. To some extent, the same constituent monosaccharides and a proportion of the linkage patterns occurred in all four fractions, indicating the fractions contained a spectrum of highly heterogeneous but structurally related polysaccharides. Several carbohydrates were enriched in specific fractions. A soluble, partially substituted, 3-linked galactan was slightly enriched in the SCM. The WW fraction was highly enriched in 3-linked glucan, presumably derived from chrysolaminaran. Chemical and AFM data for the WW and HB fractions indicated that compositional differences were associated with substantial changes in the morphology and properties of the cell surface mucilage. Soluble polymers relatively enriched in fucose conferred a degree of softness and compressibility to the mucilage, whereas most of the mucilage comprised firmer more gelatinous polymers comparatively enriched in rhamnose. The frustule residue dissolved during extraction with NaOH, and a partially substituted 3-linked mannan, together with relatively large amounts of protein, was obtained.