Polarity-wise successive solvent extraction of Scenedesmus obliquus biomass and characterization of the crude extracts for broad-spectrum antibacterial activity

The present research aims to exploit the antioxidant and antibacterial activities of freshwater microalgae, Scenedesmus obliquus , by performing polarity-wise successive solvent extraction by hexane, chloroform, ethyl acetate, acetone, methanol, and water. The antibacterial and antioxidant activities of these crude extracts were evaluated, and a correlation was established between these two parameters. The methanol and water extracts showed the highest phenolic (18–23 mg GAE/g EW) and flavonoid (5–8 mg QE/g EW) contents, resulting in a maximum antioxidant activity (IC 50 = 61–76 μg/mL for DPPH and 60–75 μg/mL for ABTS), but a lower antibacterial activity (MIC = 250–1000 μg/mL). In contrast, the hexane and chloroform extracts showed lower content of phenolics (0.3–0.8 mg GAE/g EW), resulting in a very low antioxidant activity (IC 50 = 223–278 μg/mL for DPPH and 172–220 μg/mL for ABTS). The hexane and chloroform extracts contained both saturated and polyunsaturated fatty acids with carotenoids (4–6.2 mg/g DE) and lipids as their main constituents. Flavonoids like myricetin (0.3–0.56 μg/mg) and rutin (0.24–0.75 μg/mg) were identified and extracted from this algal strain for the first time. The hexane and chloroform extracts were effective on both Gram-positive and Gram-negative bacteria, exhibiting the highest antibacterial activity (MIC = 15.6–125 μg/mL). Overall, the polar solvent extracts of S. obliquus were found to contain a high amount of phenolics and flavonoids than most reported microalgae species, making it a potential strain for developing industrially relevant biopharmaceuticals.