Selective antibacterial activity of crystalline extracellular polysaccharide extracted from isolated microalgae ( <i>Asterarcys quadricellulare</i> )

Microalgae-derived extracellular polysaccharides (EPSs) have emerged as a promising biological macromolecule for its application in pharmaceutical and biotechnological fields due to its diverse physicochemical and biological activities. In this study, an isolated microalgae (<i>Asterarcys quadricellulare</i>) was cultured in photobioreactor under the light condition by maintaining 14:10 photoperiod to produce EPSs. The yield of EPSs was found 0.392 ± 0.027 gL^-1. The biochemical analysis of extracted EPSs had shown the presence of uronic acid (0.42 ± 0.05%), phosphate (0.312 ± 0.013%), sulfate (0.531 ± 0.04%), and carbohydrates (60.33 ± 0.02%) containing glucose and galactose with a ratio of 3.24:1. FTIR analysis of EPSs revealed characteristic glycosidic bond between sugar monomer at 880 and 834 cm^-1. Therefore, extracted EPS was a complex heteropolysaccharide structure. Surface morphology of EPSs revealed the presence of a large number of symmetrical and regular filamentous repeating units. XRD analysis had shown the crystalline nature of extracted EPSs with crystallinity index of 51.73%. Antimicrobial activity of EPSs was evaluated against <i>Bacillus subtilis</i> (gram positive) and <i>Escherichia coli</i> (gram negative) bacteria. <i>Escherichia coli</i> had showed 46.67 ± 0.002% growth inhibition and 33.3 ± 2.5% (<i>P</i> < 0.01) reduction in the viable cell count compared to the negative control. Results demonstrated the promise of microalgal EPSs in the biotechnology and pharmaceutical sectors for selective antimicrobial application.