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 (IC50 = 61-76 mu g/mL for DPPH and 60-75 mu g/mL for ABTS), but a lower antibacterial activity (MIC = 250-1000 mu g/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 (IC50 = 223-278 mu g/mL for DPPH and 172-220 mu g/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 mu g/mg) and rutin (0.24-0.75 mu g/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 mu g/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.

Automated summary

The present research focused on the antioxidant and antibacterial activities of Scenedesmus obliquus, a freshwater microalgae. Different polar solvent extracts were evaluated, and the methanol and water extracts showed the highest phenolic and flavonoid contents, resulting in the strongest antioxidant activity. On the other hand, the hexane and chloroform extracts exhibited lower phenolic content and antioxidant activity, but higher antibacterial activity. Flavonoids were identified and extracted from this algal strain for the first time.