Quantitative estimation of vegetative and stress forms of Spirulina platensis and Scenedesmus obliquus, as well as a vegetative form of Dunaliella salina, revealed that S. obliquus constituted the highest polysaccharide content than other tested microalgae. The isolated polysaccharides characterized as heterogeneous polysaccharides bounded protein by FT-IR, GLC, and Elemental Microanalysis. These polysaccharides constituted of 47-66% of sugar and 14.88-41.06% of protein contents whereas galactose, mannose, glucose, and rhamnose were represented as predominant sugar in isolated polysaccharides. The isolated polysaccharides were evaluated in vitro as antiviral, cytotoxic, antioxidant and anti-cholinesterase properties. The non-toxic dose of isolated polysaccharides on Huh 7.5, MA104, BGM, and Vero cell lines were determined. The S. platensis (CEM and HEM) polysaccharides have promising antiviral, which reduced replication up to 50-87.6% of HCV genotype 4a replicon, coxsackievirus B4, rotavirus and herpes simplex type 1 virus at nontoxic doses 1.8 and 1.5 mg/ml, respectively. Furthermore, the isolated polysaccharides were assessed for in-vitro cytotoxic activity against MCF-7, HepG2, and HCT116 cell lines. The cytotoxic activity revealed that D. salina HEM polysaccharide show potent cytotoxic activity against HCT 116 cell line with IC50 64.2 μg/mL. Additionally, the isolated polysaccharides showed DPPH• scavenging activity in a dose-dependent relationship and D. salina HEM and S. obliquus CEM showed the significantly highest activity (308.16 and 308.69%, respectively) at 100 μg/mL. Furthermore, S. obliquus CEM and HEM polysaccharides exhibited the significant highest cholinesterase % inhibition activity. Microalgal polysaccharides have great therapeutically potential in drug development used as antiviral, antitumor, antioxidant and anticholinesterase agents in near future.
