**1. Introduction**

214 The Complex World of Polysaccharides

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*Zymomonas mobilis*. Bioscience, Biotechnology and Biochemistry, 56, 1335–1337. Yoo, S. H., Yoon, E. J., Cha, J., & Lee, H. G. (2004). Antitumor activity of levan 536 polysaccharides from selected microorganisms. International Journal of Biological

> Lichens are symbiotic organisms composed of a fungal partner, the mycobiont, and one or more photosynthetic partners, the photobiont, which may be either a green alga or a cyanobacterium (Nash III, 1996; Wolseley and Aguirre-Hudson, 1994 and Yoshimura, 2004). The mycobiont in this combination is an ascomycete or a basidiomycete and the photobiont a green alga or a cyanobacterium. Some lichen species can contain more than one green algal species as photobionts (Friedl and Gärtner, 1988; Friedl, 1989; Ihda *et al*., 1993). At present, about 13,500 fungal species have been recognized to be involved in lichen symbiosis (Hawksworth *et al*. 1995; Kirk *et al*., 2001). However, Sipman and Aptroot (2001) stated that this number could be as high as 20,000 after including "orphaned" species; according to Lumbsch *et al*., (2011), about 10,000 of lichenized fungi have been estimated as undescribed species.

> The hyphae of mycobionts are septate, branched, thin or thick walled, and the walls are colourless or variously coloured. It is necessary that the photobiont, which is essentially aquatic in nature, remains protected from desiccation in a lichenized terrestrial condition. The protection is provided by the mycobiont which forms the bulk of the thallus, by the development of specialized hyphal tissues in the form of a cover or cortex over the stratum of the photobiont. The development of the cortex is assumed to be stimulated by the photobiont (Ahmadjian, 1987). In some gelatinous lichens with cyanobacteria, the polysaccharidic sheath produced by the photobiont (cyanobacteria) contributes to water retention (Prieto *et al*., 2008). 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).
