A fungus-type β-galactofuranan in the cultivatedTrebouxiaphotobiont of the lichenRamalina gracilis

Abstract

A structural characterization of polysaccharides extracted from the aposymbiotically cultured photobiont of the lichen Ramalina gracilis was carried out in order to compare them with those previously found in the symbiotic thallus. The photobiont was isolated from thallus fragments, following the method of Yamamoto, and cultivated in a liquid nutrient medium. Freeze-dried cells were defatted, and the polysaccharides extracted successively with water and aq. 10% KOH, each at 100 degrees C. After purification, the soluble fractions provided a polysaccharide containing a (1-->5)-linked beta-galactofuranosyl backbone, substituted in a small proportion at O-6 by beta-Galf units. Amylose was also found, as insoluble material obtained on freeze-thawing of the alkaline extract. These polysaccharides have not been found in the symbiotic thallus of Ramalina gracilis, which contained only water-soluble (isolichenan) and insoluble glucans (nigeran and laminaran), and galactomannan. Surprisingly, the galactofuranan has similarities with those found in some fungal cell walls.

First report on polysaccharides of Asterochloris and their potential role in the lichen symbiosis

“…Thalli of C. confusa were obtained from lichens growing on sandy soil, sea level, Ilha do Mel (25 • The isolation of the photobiont and the production of biomass were obtained according Cordeiro et al [4]. Briefly, photobiont cultures were obtained from thallus fragments, using the tissue culture method of Yamamoto [5].…”

“…of the lichen Ramalina gracilis provided a fungus-type ␤-galactofuranan, which contained a (1 → 5)-linked ␤-galactofuranosyl backbone, substituted in a small proportion at O-6 by ␤-Galf units. The storage polysaccharide amylose was also found [4].…”

Lichen Polysaccharides

“…However, the recent discovery of a few additional complex heteroglycans, such as rhamnogalactofuranan (Olafsdottir, 1999), galactomannoglucans (Woranovicz-Barreira, 1999 andCarbonero et al, 2002) and thamnolan (Carbonero et al, 2005a) necessitated the closer examination of the origins of the polysaccharides. Studies carried out by Cordeiro et al, (2005), revealed that the algal partner (Trebouxia sp.) also consists of some carbohydrates such as -galactofuranan heteropolysaccharide.…”

“…Later Cordeiro et al (2005Cordeiro et al ( , 2007Cordeiro et al ( , 2008, found that, dissimilarities between the polysaccharides extracted from the cultivated photobionts Trebouxia and Asterochloris with those from their respective lichens could indicate that the photobiont or mycobiont was responsible for these differences. Galactoglucomannans were isolated from the mycobiont of (Cordeiro et al, 2005). Another heteropolysaccharide xylorhamnogalactofuranan was isolated from the lichen Cladina confusa.…”

“…Lichens have been used for ecological, medicinal and other economic purposes for over 100 years and these beneficial effects have been correlated to some extent with their polysaccharide content. Amongst the identified lichens so far, about 100 species have been studied for their polysaccharides and their composition (Cordeiro et al, 2005).…”