Polysaccharide Accumulation and Antioxidant Activity in Space-Mutated Spirulina platensis H11 Under Seawater Versus Freshwater Cultivation

As a microalga of significant economic value, the polysaccharide synthesis efficiency and bioactivity of Spirulina platensis are significantly influenced by the culture condition. In this study, S. platensis H11 obtained by space mutagenesis, was systematically compared to the growth characteristics, polysaccharide properties, and antioxidant activities between seawater and freshwater culture conditions. The results showed that the biomass concentration (8.87 g L−1) and polysaccharide yield (5.46 g L−1) of the strain in the seawater condition were increased by 16.1% and 14.7%, respectively, compared with that in the freshwater condition. Seawater-derived polysaccharides (SSPS) featured an α-configuration glucan backbone and exhibited higher molecular weight (466.19 kDa) and an additional β-configured anomeric signal compared with FSPS. Despite having lower contents of sulfate groups (0.56% DW) and glucuronides (4.92% DW) by 39.3% and 33.5%, respectively, compared to FSPS (0.78% sulfate group, 6.57% glucuronide), SSPS exhibited significantly enhanced hydroxyl radical-scavenging activity, demonstrated by a decreased IC50 value of 2.53 mg mL−1 as opposed to 3.07 mg mL−1. The present study confirms that seawater cultivation is an effective strategy for inducing the synthesis of highly active polysaccharides in space mutant strains, and provides a theoretical basis for the targeted production of polysaccharides in S. platensis and their functional and precise application.