Variations in the substituted 3-linked mannans closely associated with the silicified walls of diatoms

The polysaccharides from cleaned frustules of the diatoms Pinnularia viridis (Nitzsch) Ehrenberg, Craspedostauros australis Cox, Thalassiosira pseudonana Hasle et Heimdal, and Nitzschia navis-varingica Lundholm et Moestrup were extracted with hot alkali that dissolved the silica and were characterized by constituent sugar and linkage analyses. The polysaccharides from P. viridis were investigated further by permethylation, partitioning according to solubility, desulfation, and CD3I-methylation. Yields of carbohydrate in the hot alkali extracts ranged from 0.9% to 1.8% w/w based on the dry weight of the silica. Mannose was the dominant sugar in the polysaccharides from all four species (54-69 mol% of constituent sugars), although 14 other monosaccharides, including neutral sugars (glucose, galactose, xylose, arabinose, rhamnose, fucose), acidic sugars (glucuronic acid, galacturonic acid, 2-O-methylglucuronic acid), and O-methylated neutral sugars (2-O-methylrhamnose, 3-O-methylrhamnose, 2,3-di-O-methylrhamnose, 3-O-methylxylose, 4-O-methylxylose) were also detected in varying proportions among the four samples. The polysaccharides were predominantly composed of a 3-linked mannopyranose backbone with a prevalence of linkage and/or substitution at O-2 of the 3-linked mannopyranosyl residues, and they were polyanionic, bearing uronic acid residues and/or sulfate esters. There were, however, species-specific differences in the degree and position of substitution on the mannan backbone, the type and substitution patterns of the anionic substituents, and the type and linkage patterns of sugars other than mannose. Although definitive functions for these polysaccharides in diatom biology remain uncertain, a possible role in biosilicification is discussed.