Algal Polysaccharides: Properties and Applications

Bioactive polymers mainly polysaccharides such as cellulose, chitin, amylose and beta-glucan are well discovered from various sources in nature. Amongst chitin, as the second most abundant polysaccharides after cellulose, is known as a highly insoluble polysaccharide consisting of β-4-linked N-acetyl-D-glucosamine, which has been identified as a structural component of crustaceans, crabs, shrimps, insects, and other arthropods, as well as a component of the cell walls of fungi [1] and some alga [2]. Although various bioactive functions are characterized well, less is known about the relationship between structure of biopolymers and their physiological/biological activities. Therefore, more efficient utilization of these raw materials is being required in various fields like in pharmaceutical, medical, agricultural and industrial applications. Since biopolymers are interesting in various applications described above, especially marine alga are of focusing as the plentiful sources for various bioactive compounds including proteins, poly-phenolic compounds, carotenoids and polysaccharides, which was identified as cell wall structural or intercellular components. Structural specificities, substitution patterns and polymerization degrees of polysaccharides are prerequisites of physical and functional properties in alga, even though other biopolymers such as proteoglycans, polymeric phenolics and proteins may participate in the synergic activities and the formation of algal cell wall. Great variability of cell wall polysaccharides in marine alga has been identified to be involved in the determination of the species or taxa, and developmental stages of life-cycle. In general, alga have been grouped into mainly Clorophyta (green algae), Rhodophyta (red algae) and Phaephyta (brown algae). These alga produce various bioactive compounds including sulphated polysaccharides as in complex composite cell walls consisting of cellulose, sulfated galactans, xylan or mannan fibrils. For instance, green algae of Ulvophyceae consists of sulphated (1→3)-β-D-galactans [3] and sulfated glucuronofucan containing both fucofuranose and fucopyranose residues were identified from the brown alga Chordaria flagelliformis [4]. As described above, it is very hard to identify their structural components in precise. In addition, more complex compounds which were highly branched sulphated hetero-polysaccharides of brown alga were assigned from many different origins. Eventually, both sulphated and carboxylated algal polysaccharides are known to exhibit biological activities such as anti-herpetic [5], anti-coagulant [5,6], antiinflammatory [7], anti-tumor [8], anti-microbial [9], immunemodulatory [10], anti-viral activities [11] and etc. Among them, immuno-modulating and anti-coagulant activities seem to be the most potent biological activities for understanding the mechanism of action of algal polysaccharides [5,6,10,12]. Therefore, the geometry of glycosidic linkages to get better understanding for physical or chemical properties of algal polysaccharides is critically important. Besides marine algal cell wall polysaccharides, in recent, alpha-amylose like polysaccharide consisting of glucose mainly was identified from a photosynthetic microalgae Dunaliella tertiolecta [13], which was isolated as a biodiesel producer. It can be effectively converted into glucose by enzymatic or acidic hydrolysis, which recovered over 90% of glucose from the defatted cell wall. High potential production of algal biomass and efficient conversion to glucose may allow further exploration for industrial exploitation of bio-ethanol. It suggested that micro-algal defatted biomass can provide the significant commercial potential to increase net ethanol production as an alternative bioresource replaceable of corn or edible starch. Therefore polysaccharide derived from the defatted biomass of D. tertiolecta and characterized as a homo-polysaccharide consisting of glucose is a promising candidate for industrial exploitation for ethanol production in aspect of bio-refinery [13]. According to many reports, polysaccharides obtained in homoor hetero complexes from many different kinds of alga including photosynthetic microalga and other species have great potential for bioactive materials, bio-ethanol and medicinal applications, because of their biocompatibility with the human tissues, biodegradability and non-toxicity.