Evaluation of antioxidant and antibacterial activities of some selected seaweeds from Saint Martin's Island of Bangladesh

•Antioxidant and antibacterial activity of seaweed crude extracts were evaluated.•The most active compounds from seaweeds were extracted with methanol and acetone.•Methanolic extract of seaweeds showed better antioxidant activity.•Methanolic extracts of brown seaweeds inhibited bacterial growth.•Brown seaweed can be used as natural additives in the food processing industry. The effects of extracting solvents on the antioxidant and antibacterial activity of crude extracts of six seaweeds were investigated. The antioxidant activity of seaweed crude extracts was evaluated by total antioxidant activity (TAA), 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt (ABTS), and cupric reducing antioxidant capacity (CUPRAC) assays. Acetone extract of B. wrightii showed significantly (P < 0.05) the highest total phenolic content (TPC) (71.09 mg GAE/g), while methanolic extract of Porphyra sp. had the highest total flavonoid content (75.61 mg QE/g). Methanolic extract of S. muticum showed significantly (P < 0.05) the highest TAA. Methanolic extract of P. tetrastromatica and S. muticum showed significantly (P < 0.05) the highest DPPH and ABTS radical scavenging effects, while acetone extract of B. wrightii showed significantly the highest ECA0.50 value (0.78 mg/ml) of CUPRAC activity. Pearson's correlation test and principal component analysis (PCA) confirmed the relationship between TPC and various antioxidant activity assays of seaweed extracts. Methanolic extracts of P. tetrastromatica, S. muticum and H. clathratus showed higher inhibitory zones against all the bacterial strains than other seaweed extracts. The results of this study indicated that brown seaweeds showed comparatively better antioxidant and antibacterial activity, which could be utilized in the food processing industries.