Elucidation of polysaccharide origin in Ramalina peruviana symbiosis

A structural elucidation of polysaccharides extracted from the aposymbiotically cultured mycobiont of the lichen Ramalina peruviana was carried out in order to determine whether the polysaccharides found previously in the symbiotic thalli are produced by the mycobiont or photobiont or both. The mycobiont isolate was cultivated on a solid malt-yeast extract-medium and the freeze-dried colonies were defatted and the polysaccharides extracted successively with hot water and aq. 2% KOH, each at 100 degrees C. The alkaline extract was obtained in much higher yield (31.5%) and submitted to a freeze-thawing treatment, giving rise to a precipitate (PK2) of a mixture of (1-->3),(1-->4)-alpha-glucan (1.2:1 ratio, nigeran) and a (1-->3)-beta-glucan (laminaran). The mother liquor was treated with Fehling solution to give a precipitate (galactomannan). This had a (1-->6)-linked alpha-d-mannopyranosyl main chain, substituted at O-4 and in small proportion at O-2,4 by beta-Galp units. All three polysaccharides have previously been found in the symbiotic thalli of R. peruviana, showing that these are produced by the fungus, without the participation of the Trebouxia photobiont. Surprisingly, isolichenan, a cold-water soluble (1-->3),(1-->4)-alpha-linked-glucan (3:1 ratio) was not found in the isolated mycobiont, despite being the main polysaccharide found in the thalli.