Carbohydrate, glycolipid, and lipid components from the photobiont ( Scytonema sp.) of the lichen, Dictyomema glabratum

Abstract

The photobiont of the lichen, Dictyonema glabratum ( Scytonema sp.), was isolated and cultivated in a soil-extract medium and submitted to chemical analysis. Successive extractions with CHCl 3–MeOH, aqueous MeOH, and H 2O gave rise to solutions of lipids (25%), low-molecular-weight carbohydrates (22%), and polysaccharides (4%), respectively. TLC of the lipid extract showed the presence of glycolipids, which were further purified and examined by NMR spectroscopy and GC–MS. Monogalactosyldiacylglycerol (1%), digalactosyldiacylglycerol (0.8%), trigalactosyldiacylglycerol (0.4%), and sulfoquinovosyldiacylglycerol (0.5%) were identified. The most abundant fatty acid ester in each fraction was palmitic (C 16:0), but a great variation of the ester composition from one to another was found. Others present were those of C 12:0, C 14:0, C 15:0, C 16:1, C 17:0, C 18:0, C 18:1, C 18:2, C 18:3, C 22:0, C 22:2, and C 24:0. The lipid extract was also subjected to acid methanolysis, which gave rise to dodecane, 2-Me-heptadecane, 2,6-Me 2-octadecane, and 8-Me-octadecane, methyl esters of C 14:0, C 15:0, C 16:0, C 16:1, C 17:0, C 18:0, C 18:1, C 18:2, C 20:0, and C 24:0 fatty acids, and the dimethyl ester of decanedioic acid. The polysaccharide had mainly Glc, Gal, and Man, with small amounts of 3- O-methylrhamnose and 2- O-methylxylose, both found in plants, and unexpectedly, some of the units were β-galactofuranose, typical of fungal, but not cyanobacterial polysaccharides. The low-molecular-weight carbohydrates showed mannose as the main free reducing sugar, which differs from Nostoc sp. and Trebouxia sp. photobionts.