**5. Biological activities of lichen polysaccharides**

220 The Complex World of Polysaccharides

(Hawksworth, 1982).

**4. Polysaccharides as a taxonomic tool** 

Colleman is a complex heteroglycan containing the unusual monosaccharides 2-OMe Man*p* and 2-OMe-Arap as well as Xyl*p* and Gl*cpA* (Jensen *et al*., 2010). The presence of uronic acids has been reported previously from cyanobacterial polysaccharides. Since the structural features and sugar content of colleman is representative of polysaccharides of cyanobacterial origin, it is proposed that colleman originates from the cyanobacterial partner. Ruthes *et al*. (2010) were able to isolate (1→4)-linked β-D-xylan (an EPS) and heteropolysaccharide with a complex structure of β-L-Ara*p* and β-D-Xyl*p*-(1→4)-linked units from *Peltigera canina*. Again it was opined that the photobiont, *Nostoc muscorum* was the source of this heteropolysaccharide.

The identification and classification is generally based on morphology of the organism. Taxonomy of lichen species have been corroborated by phylogenetic applications with the advances in DNA technology. Lichen polysaccharides have been used as a taxonomic tool and chemotaxonomic classification has resulted in clarification of conflicting taxonomic data.

The lichen-forming ascomycete order Lichinales comprises around 250 species and is distributed among 52 genera and four families (Eriksson, 2006). Earlier molecular studies (Wedin *et al*., 2005) did not confirm its phylogenetic relationships, although the order was treated as a separate class, *Lichinomycetes* (Hibbett *et al*., 2007). Since alkali and water-soluble polysaccharides from *Lichina pygmaea* and *L. confinis* reflect phylogeny in other ascomycetes (Prieto *et al*. 2008), an isolated polysaccharide was purified to investigate whether such polysaccharide compounds in the Lichinomycetes are distinctive. Results support molecular studies showing that lichen species are remote from Lecanoromycetes as the galactofuranose residues are in the α-configuration. That the Lichinomycetes were part of an ancestral lichenized group cannot be established from the present data because the extracted polysaccharide does not have the galactofuranose residue in the β configuration; however, the data suggests that an ancestor of the Lichinomycetes contained a mannan and was part of an early radiation in the ascomycetes. Polysaccharides present support the molecular data obtained recently that the Lichinales are distinct from other ascomycete groups and should be treated as a separate order in the separate class Lichenomycetes (Reeb *et al*., 2004). However, more representatives in the order must be subjected to molecular studies and more polysaccharides be investigated before confirming this hypothesis. Interestingly, the relative basal position of *Lichina* with respect to the water soluble polysaccharides agrees with the suggestion that ancestral lichens contained cyanobacteria as the photosynthetic partner

Investigation of mannose containing polysaccharides as a taxonomic tool centers around the structural diversity of the galactomannans isolated from several lichenized fungi. The taxonomic value of these galactomannans depends on the side-chain substituents on (1 →6)-

Although classical taxonomy regarded *Cladina* as sub-species of *Cladonia,* lichenologists considered them to be distinct species. It was shown that galactomannans are important chemotypes in determining the taxonomy of *Clodonia* spp. and other related genera

linked α-D-mannopyranosyl main chains (Gorin and Lacomini, 1985).

Many lichens are known to have immunomodulating properties, potent antibiotic, antitumour, antiviral as well as antioxidant properties which are mostly attributed to the secondary metabolites (Malhotra *et al.,*2008, Behera, *et al*., 2007). According to Yanaki *et al*., (1986) and Bohn and BeMiller, (1995), functional activity of polysaccharides mainly depends on molecular weight, degree of branching, water solubility, structure and configuration. Hence they have different uses in different fields. The biological activities of lichen polysaccharides reported have been limited to anti-tumor, anti-inflamatory or immunomodulatory activity (Omarsdottir *et al*., 2007).

Cordeiro *et al*., (2008) reported that one of the β-galactofuranan polysaccharides isolated from *Trebouxia* sp., the algal symbiont of the lichen *Ramalina gracilis* expressed *in vitro* activity on peritoneal macrophages. Further studies carried out on 4 lichen polysaccharides by Omarsdottir, *et al*., (2006) showed that, three heteroglycans namely Ths-4, Ths-5 and thamnolan and a β-glucan (Th-2) isolated from the lichen *Thamnolia vermicularis* var. subuliformis showed an effect on the human immune system. Thamnolan, a galactofuranorhamnan had less mitogenic effect than Ths-5 and Ths-2 indicating that its unusual galactofuranorhamnan structure may be responsible for its different immunomodulatory activity. In a study on the immunomodulatory activities of an aqueous lichen extract from *Cetraria islandica* , Freysdottir *et al.,* (2007) discovered that the extract was able to upregulate IL-10 secretion. Interestingly, when the individual components of this extract (lichenan and isolichenan and secondary metabolites protolichesterinic and fumarprotocetraric acids) were subjected to the same assay, only lichenan displayed antiinflammatory effects (Freysdottir, *et al.,* 2008).

The cytotoxic activity, phagocytic activity and antitumor activity of an α-D glucan from *Ramalina celastri* has been reported (Leão *et al.,* 1997 and Stuelp-Campelo *et al*., 2002). Based on this, De Araújo *et al.,* (2011) found that a sulphated -D glucan lichen polysaccharide extracted from *Ramalina celastri* exhibited antischistosomal activity. These -D glucans with andlinkages are linear and water soluble and are known for their ability to stimulate the mononuclear phagocyte system and improve host resistance to viral, bacterial and parasitic infections.

According to Nishikawa *et al*., (1970), O-acetylated pustulan isolated from three species of *Umbilicaria* showed a significant antitumor effect against the implanted Sarcoma-180 in mice. The polysaccharide fractions isolated from six species of lichens were studied for

antitumor activity (Nishikawa *et al*., 1974). The active polysaccharide present in *Lasallia pensylvanica,* was the partially O-acetylated (1-6) β-glucan, pustulan while that of *Usnea rubescens,* was identified as a lichenan type polysaccharide. The other 4 polysaccharides were from the *Cladonia* sp*.*; *Cladonia crispata, Cladonia rangiferina* subsp. *grisea, Cladonia mitis*  and *Cladonia squamosa* all of which showed significant antitumor activity. These *Cladonia* polysaccharides which were complex heteroglycans of the galactomannan type were found to exert moderate antitumor activity. However, previous studies have shown that the isolichenin type polysaccharides were not as effective in their antitumour effect. In another study, isolichenin isolated from the lichen *Usnea fasciata* showed moderate activity against Ehrlich tumor cells (Periera *et al*., 1994). In 1989, Hirabayashi *et al*., showed inhibitory effect of a lichen polysaccharide sulfate (GE-3-S), isolated from the lichen *Umbilicaria esculenta* on the replication of human immunodeficiency virus (HIV) in vitro.

Behera *et al*., (2007) showed correlation between lichen protein/polysaccharide ratio and their antioxidant properties. Since cultured lichen extracts were used in the study, the effect of secondary metabolites as well as polyphenols present in the extract have not been evaluated. Thus the antioxidant activity cannot be solely attributed to the polysaccharide specially since polyphenols are known antioxidants. In addition, the mechanism of scavenging activity of polysaccharides on free radicals is not fully understood yet.
