## 2 Biologically Active Polysaccharides--Structure and Function
### Algal and Lichen Polysaccharides  
Rather for practical reasons than according to the official taxonomy, algae, a complex group of photosynthetic organisms, are divided into multicellular marine organisms (red--Rhodophyta, brown--Phaeophyceae, and green--Chlorophyta, algae) and unicellular microalgae [60]. In general, the polysaccharides are the main components of algal biomass, and in contrast to terrestrial plants, the polysaccharides of all the marine algae are sulfated [61]. Due to the large number of biologically active algal polysaccharides, only a few examples of structures, mainly with immunomodulatory and anti-tumor activity, will be described in this short section. This arbitrary choice is due to the main interest in the synergism of selenium and such polysaccharides. Fucoidan is a polysaccharide isolated from various species of brown algae (Phaeophyceae), with a high pharmacological potential [62]: immunostaining, anticancer, anticoagulant, antiviral, antibiotic, anti-inflammatory, and antioxidant activity [63; 64]. The biological activity of fucoidan depends on the organisms from which it is isolated. It has been proven that _Fucus vesicolus_ polysaccharides inhibit the production of TNF-\(\alpha\), but fucoidans isolated from _Undaria pinnatifida_ induced TNF-\(\alpha\), showing cytotoxic activity against a tumor cell line [65]. Additionally, fucoidans show activity towards bone marrow mononuclear cells (BMCs), protecting them against apoptosis [66]. There are two types of fucoidan molecules, containing a backbone of (1-3)-linked \(\alpha\)-L-fucopyranose residues or (1-3)-linked \(\alpha\)-L-fucopyranose and (1-4)-\(\alpha\)-linked-L-fucopyranose residues [67]. Additionally, the molecules may be modified by linking with sulfate (VI) residues [68]. The structure of fucoidans includes monosaccharides such as fucose, galactose, glucuronic acid, mannose, or glucose [69]. Laminarans are 1,3-\(\beta\)-D-glucans, isolated from the brown algae _Saccharina cichorioides_, that have proven anti-cancer activity. Laminarans in combination with radiation therapy may have a positive effect on the treatment of human breast cancer [70]. Polysaccharides isolated from _Sargassum latifolium_ have a specific cytotoxic activity against lymphoblastic leukemia [71]. Further sulfonated polysaccharides isolated from red seaweed _Champia feldmannii_ have been shown to inhibit the development of sarcoma, in studies carried out with mice [72]. Moreover, carrageenanans--sulfonated galactans derived from _Chondrus ocellatus_, linear high-molecular-weight water-soluble D-galactans, consisting of alternately arranged units of \(\alpha\)-D-galactopyranose and \(\beta\)-D-galactopyranose, linked by 1,3- and 1,4-glycosidic bonds, increased the antitumor effect of fluoroarcacil [73]. Since the high molecular weight of the native lambda-carrageenanans decreases their solubility and limits their bioactivities, [PERSON] et al. obtained microwave-degraded low-molecular weight lambda-carrageenan, which could add the antitumor activities of 5-fluorouracilacil and improve the immunocompetence damaged by this chemotherapeutic [74]. One of the best-known representatives of the sulfonated polysaccharides, isolated from red algae, is porphyrin isolated from _Porphyna capensis_ or _Porphyna huinensis_. A typical porphyrin chain consists of alternating \(\beta\)-D-galactose, \(\alpha\)-L-galactose-6-sulfate, and 3,6-dehydro-\(\alpha\)-galactose units. Other sulfonated polysaccharides with similar structure, agarans, were isolated from _Polysiphonia stricissima_, _Polysiphonia nigscissoides_, _Polysiphonia nigscress_, and _Polysiphonia atterima_; however, the agaran backbone may be interrupted by different O-linked substitutions in addition to sulfate, including methyl andxyl groups. Sources of such polymers are also _Acanthophora spiciifera_ and _Gracilaratica_[75; 76]. The major sulfonated polysaccharide isolated from green algae is ulvan.