Cultivation of Spirulina platensis under salt stress conditions (0.02 M as control), 0.04 and 0.08 M NaCl led to a remarkable alteration of algal metabolism as well as an enhancement or induction of biologically active compounds. Concerning algal growth, salt stress caused a decrease in dry weight, chlorophyll a content as well as certain xanthophylls (neoxanthin and violaxanthin) while β-carotene production was stimulated especially at higher salt concentrations. Biochemical analysis of salt stressed algal revealed that lipid content was slightly increased together with certain saturated and unsaturated fatty acids especially the polyunsaturated ones (γ-inolenic acid, omega 3 fatty acid). Electrophoretic analysis of soluble protein pointed out that certain high molecular weight protein bands were not detected comparing with the protein marker. Five new protein bands of molecular weights 190, 158, 113, 77 and 28 KDa were recorded, in addition to an increase in the intensity of 6 already existing bands. Phosphate buffer and water extracts of the algal exhibited antiviral activities against both Hepatitis-A-virus-type-MBB (HAV-MBB strain, RNA virus) and Herpes simplex-virus-type-1 (HSV-1, DNA virus). Water extracts were found to be more effective than phosphate buffer extracts in inducing antiviral activities (98%) especially against HSV-1 virus. The same water extract of the salt stressed algal demonstrated higher anticoagulating activity compared with those of heparin and the positive control measured by clotting time assay. Antioxidant activity of the algal successive extracts against 2, 2 diphenyl-1-picrylhydrazyl and 2,2'- azino-bis (ethylbenzthiazoline-6- sulfonic acid) radical methods revealed moderate antioxidant activity of the non-polar algal extracts (petroleum ether) which were doubled with increasing extract concentration. The lowest activity was recorded by the partially polar (ethyl acetate) algal extract of both concentrations at all salinity levels. While the polar extracts (ethanol and water) showed higher antioxidant activities which were doubled with increasing extract concentration. Ethanolic algal extract (100 µg/ ml at 0.08 M NaCl) exhibited the highest antioxidant activity compared with those of the synthetic antioxidant butylated hydroxy anisol as standard (85.0, 89.9 and 86.0, 91.8% respectively).
Key words: Spirulina platensis, antioxidant-antivirus, biochemical studies, salt stress.