Solvent-Dependent Antioxidant and Antimicrobial Activities of Chlorella sp. and Scenedesmus sp. Extracts from Freshwater Sources

The aim of this study was to comprehensively evaluate the antioxidant and antimicrobial properties of extracts obtained using different solvents (hexane, acetone and methanol) from two freshwater green microalgae species Chlorella sp. and Scenedesmus sp., commonly found in Türkiye. In addition to antioxidant and antimicrobial assays, total phenolic content (TPC), total flavonoid content (TFC), chlorophyll (Chl a and b) and total carotenoid levels were quantitatively determined. A qualitative phytochemical screening was also conducted to detect the presence of alkaloids, tannins, saponins, terpenoids, steroids and glycosides. Antioxidant activity was assessed using DPPH radical scavenging and CUPRAC reducing power assays. The three solvents of different polarity, methanol extract was the most efficient and had the highest extraction efficiency (17%). The hexane extract of Scenedesmus sp. showed the highest DPPH scavenging activity (lowest IC₅₀), while the methanol extract of Chlorella sp. exhibited the strongest reducing power in the CUPRAC assay. Antimicrobial activity, evaluated using the disc diffusion method, revealed that the acetone and methanol extracts showed significant antimicrobial activity, particularly against E. coli, K. pneumoniae and Candida albicans. The highest TPC values were observed in the methanol extracts: 55.95 mg GAE/g for Chlorella sp. and 70.24 mg GAE/g for Scenedesmus sp. Similarly, the methanol extract of Chlorella sp. exhibited the highest TFC (73.41 mg QE/g), while Scenedesmus sp. showed a lower flavonoid content (16.16 mg QE/g). Chlorophyll levels were relatively similar across all solvent extracts. These findings highlight the potential of Chlorella and Scenedesmus species as valuable sources of natural antioxidants and antimicrobial agents, with solvent type significantly influencing their bioactive compound profiles and biological activities.