Nature’s Marine Defenders: Evaluating the Antioxidant Richness and Antibacterial Potential of Brown Seaweeds Sargassum polycystum and Cystoseira tamariscifolia

Seaweeds represent a diverse reservoir of marine macroalgae rich in phytochemicals with significant therapeutic and biotechnological potential. As interest grows in sustainable bioactive compounds for biotechnological and pharmaceutical use, this study provides a comparative evaluation of the phytochemical profiles, antioxidant capacities, and antibacterial potential of ethanolic extracts of Sargassum polycystum (SP) and Cystoseira tamariscifolia (CT). Phytochemical analysis revealed distinct profiles, with SP showing higher total phenolic (65.18 ± 0.06 mg GAE/g) and tannin (53.73 ± 0.02 mg TAE/g) contents, whereas CT was enriched with flavonoids (55.26 ± 0.06 mg QE/g). Both extracts exhibited concentration‐dependent antioxidant effects across glutathione (GSH) levels, superoxide dismutase (SOD) activity, and catalase (CAT) assays. SP demonstrated superior broad‐spectrum antioxidant potential, evidenced by elevated GSH levels and more potent SOD inhibition (estimated EC 50 : 11.43 μg/mL) than CT (estimated EC 50 : 22.86 μg/mL). In contrast, CT exhibited a highly selective antioxidant profile with superior CAT‐mediated activity (estimated EC 50 : 1.90 μg/mL), reflecting its distinct phytochemical profile. In antibacterial assays, CT displayed significantly higher efficacy against E. coli (ATCC 25922) and clinical isolates, yielding larger zones of inhibition (19.5 and 20 mm) and a lower minimum inhibitory concentration (MIC: 250.05 μg/mL) compared to SP (14 and 15 mm; MIC: 583.55 μg/mL). Mechanistic studies showed that CT induced significant membrane damage in E. coli (ATCC 25922), evidenced by a substantial increase in extracellular protein leakage from 1.40 to 5.42 mg/mL at a concentration of 500.01 μg/mL. Microscopic observations corroborated these findings, revealing that CT triggered more rapid and severe structural alterations than SP, underscoring its superior membrane‐disruptive and bactericidal activity against E. coli strains. Collectively, these results identify SP as a potent broad‐spectrum antioxidant and CT as a high‐efficacy antibacterial agent with selective enzymatic antioxidant properties. Both macroalgae demonstrate substantial efficacy as natural reservoirs of bioactive secondary metabolites possessing potent antioxidant and antibacterial activities, offering promising scaffolds for the development of novel therapeutics against drug‐resistant E. coli infections.