Evaluation of antioxidant activity and phenolic profile of filamentous terrestrial cyanobacterial strains isolated from forest ecosystem

The present study aims to investigate antioxidant activity and the phenolic content in five terrestrial cyanobacterial strains isolated from the Fruška Gora mountain forest ecosystem (Serbia). The antioxidant activity of crude ethanolic extracts was assessed using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing ability of plasma (FRAP) assay. All the tested cyanobacterial strains belonging to Phormidium, Nostoc, Oscillatoria, Anabaena, and Calothrix genera exhibited antioxidant activity. The ethanolic extracts from Calothrix M2 showed strong inhibition against 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals (half maximal inhibitory concentration (IC₅₀) = 30.72 ± 3.31 μg mL⁻¹) whereby Phormidium M1 expressed the highest radical potential in FRAP assay (22.48 ± 2.18 mg ascorbic acid equivalents (AAE) g⁻¹). Furthermore, the presence and content of 15 phenolics in ethanolic extracts of the tested cyanobacterial strains were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The richest strain in polyphenolics was Phormidium M1 which contained 12 compounds, mostly phenolic acids and flavonoid glycosides, followed by Nostoc M1 and Oscillatoria M2. The noticeable amount of phenolic acids (quinic acid 502 μg g⁻¹; gallic acid 84.9 μg g⁻¹; vanillic acid 50 μg g⁻¹) was observed in Phormidium M1 indicating that phenolic compounds could contribute to the antioxidant activity observed in FRAP assay. LC-MS/MS analysis showed that the tested terrestrial cyanobacterial species could be regarded as a promising new source of bioactive phenolic compounds. Since no significant correlation was observed between phycocyanin content and antioxidant activity, other compounds present in the crude extracts contributed to antioxidant capacities in DPPH and FRAP assay. This study highlights the potential of terrestrial cyanobacterial strains inhabiting forest ecosystems as an interesting source of compounds with antioxidant potential for further researches and exploitation.