Biological activities of polysaccharides fraction from Arthrospira platensis and Nostoc muscorum cultivated under abiotic stress conditions

Background
Cyanobacteria are Gram-negative bacteria with a long evolutionary history; they are the only prokaryotes capable of performing oxygenic photosynthesis in the same way that plants do. Cyanobacteria are appealing platforms for carbon-neutral industrial processes, and they have several advantages as biotechnological hosts, such as simple growth requirements and genetic manipulation.
Objective
The aim of this study was to select Arthrospira platensis and Nostoc muscorum cultured under physical (different light intensities, absence, or presence of aeration) and chemical (sulfate concentrations) stress culture conditions and determine its effect on polysaccharides formation. Evaluate the various algal extracts’ capacity for antioxidant protection using DPPH and ABTS, anticancer activity against various cancer cell lines, and antiviral (against SARS-Cov-2) activities.
Materials and methods
A. platensis and N. muscorum were cultivated on Zarrouk and BG-11 medium, respectively. Algal species were cultured and incubated in controlled photoperiod of, 16-8 L/D cycles, light intensity of 40 µE/m/s and temperature at 25°C±2°C with continuous aeration supplied with air (60 bubbles/min). The growth rate of both selected species and their cultivation under abiotic stress conditions were assessed using optical density and dry cell weight. Polysaccharide extraction using various techniques and determination of total hydrolysable carbohydrates were performed. Evaluation of antioxidant activity of algal extracts using different methods (DPPH and ABTS). Determination of Anticancer Activity (MTT assay) against various cancer cell lines, Also the antiviral activity using MTT cytotoxicity assay against SARS-Cov-2. Silver nanoparticles (AgNps) were biosynthesized by polysaccharides extracts of both N. muscorum and A. platensis. Their characteristics were determined by spectrophotometric absorption, zeta potential and TEM.
Results and conclusion
The effects of aeration, MgSO concentrations, and light intensity were all positively correlated with the algal growth and production of polysaccharides. Ethanol was found to be highly efficient for polysaccharide extraction. In both investigated cyanobacteria species, the intracellular polysaccharide concentration was greater than the extracellularly released polysaccharide content. All extracts recorded high activity as antioxidant and anticancer which reflects a strong relationship between both activities e.g.: (N.E.H.S, N.E.No.A, N.I.L.L, and N.I.L.H) in N. muscorum and (S.E.C, S.E.S.H, S.I.C, and S.I.L.H) in A. platensis. Also, N.I.C, S.I.L.H, and S.I.S.H showed anti-SARS-CoV-2 activity at safe concentrations with high selectivity indices (SI=CC50/IC50).