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) thanU. 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.