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Within the last few decades, marine organisms have received extensive research attention from a number of scientific organizations due to their potential biological activities including antioxidant and antimicrobial properties. Among the marine species, brown seaweeds have significant role in the synthesis of a variety of bioactive chemicals. The current work was mainly focused on the screening of antimicrobial activity and antioxidant properties as well as the characterization of phenolics and flavonoids in the seaweed methanolic extract (SME) for the three brown macroalgae Cystoseria myrica, Sargassum polycystum and Turbinaria triquitra. The antimicrobial properties were tested against two gram-positive and two gram-negative bacteria as well as Candida albicans as a unicellular fungus. The obtained results revealed that all seaweed extracts showed a positive antimicrobial activity against all tested bacteria and yeast. The largest inhibition zones were recorded by the T. triquetra against Staphylococcus aureus and Salmonella typhimurium (2.2 cm) for both, although it had the least inhibitory effect against Candida albicans. Significantly, S. polycystum recorded the highest phenolics, flavonoids and carotenoids content (1392, 56 and 8 µg /ml) respectively which led to the highest antioxidant capacity. On the other hand, the most effective algal extract against yeast was C. myrica. The most dominant detected phenolic compounds in all (SME) were quercetin and apigenin which are known for their high antioxidant activity. In conclusion, all of the tested seaweed extracts exhibited considerable antimicrobial and antioxidant activities.

The potential of macroalgae as biostimulants in agriculture was proved worthy. Vicia faba and Helianthus annuus are socioeconomic crops owing to their increasing demand worldwide. In this work, we investigated the energetic role of seed presoaking and irrigation by the brown seaweed, Sargassum polycystum aqueous extract (SAE) on certain germination and growth traits, photosynthetic pigments, carbohydrates, phenolics, flavonoids, and the total antioxidant activity. Compared to the control plants, our consequences revealed that seeds that received the SAE improved all the germination and growth criteria for both crop plants. Furthermore, the SAE significantly increased the carotenoids, total photosynthetic pigments, and total carbohydrates by (14%, 7%, and 41%) for V. faba and (17%, 17%, and 38%) for H. annuus, respectively. Phenolics and flavonoids were significantly induced in Vicia but slightly promoted in Helianthu plants, whereas the total antioxidant activity in both crops non significantly elevated. Even though The NPK contents were significantly stimulated by the SAE in Vicia plants, the effect was different in Helianthus, where only nitrogen content was significantly enhanced, whereas phosphorus and potassium showed little enhancement. Thus, the SAE treatment is one of the superlative sustainable strategies for food, feed, and as excellent plant conditioner.