Growth and toxin production inhibition in Gymnodinium catenatum by extracts of two native macroalgae Cystoseira barbata and Ulva lactuca from the southern Mediterranean Sea

Macroalgae are known to release allelochemicals that can suppress the growth of their competitors in the same environment. The present study investigates the potential effects of aqueous and methanol extracts of two native macroalgae (Cystoseira barbata and Ulva lactuca) from the southern Mediterranean Sea on the growth and saxitoxin production of harmful algal bloom-forming dinoflagellate Gymnodinium catenatum. Aqueous extracts of U. lactuca and C. barbata significantly increased G. catenatum growth by 60–70% as compared to the control, but they had no discernible effect on toxin production. Conversely, the methanol extracts of the two macroalgae showed inhibitory effects on G. catenatum growth, with greater suppression occurring by C. barbata (IC₅₀ = 1.5–50 mg mL⁻¹) than U. lactuca (IC₅₀ = 3.7–214 mg mL⁻¹). The macroalgal methanol extracts also induced the toxin production in G. catenatum cells during the first 4 days of incubation, but this production had dropped to undetectable levels by the end of the experiment. G. catenatum cells treated with macroalgal methanol extracts sedimented without lysing or releasing toxins and then transformed to non-viable, nontoxic temporary cysts. Our study suggests that methanol extracts of these macroalgae could be a viable method to control harmful algal blooms in confined coastal areas. Nevertheless, ecotoxicological research on the extract’s possible effects on aquatic life is required before use.