Suppression of cyst germination success in Gymnodinium catenatum and Ostreopsis cf. ovata by macroalgal extracts from the southern Mediterranean Sea

This study examines the potential inhibitory effects of extracts of seven Mediterranean macroalgae on the germination success of two harmful dinoflagellate cysts, Gymnodinium catenatum and Ostreopsis cf. ovata. The results revealed that aqueous and methanolic extracts of these macroalgae showed varying inhibitory effects on the viability of the cyst germlings, preventing them from dividing and producing motile vegetative cells. Among macroalgae, three Cystoseira species (C. compressa, C. barbata, and C. crinita) exhibited the strongest inhibitory effects on the germination success of the two cyst types. The methanolic extracts of these species showed higher inhibitory effects on O. cf. ovata cysts (% inhibition = 88%–100%) than G. catenatum cysts (83%–95%). Based on the median inhibitory concentration (IC₅₀), the methanolic extracts of these macroalgae have exhibited stronger inhibitory effects on germling viability (IC₅₀ = 0.05–1.5 mg extract g⁻¹ sediment) than aqueous extracts (IC₅₀ = 0.9–590 mg extract g⁻¹ sediment). The study suggests that macroalgal materials, specifically Cystoseira species, would be a promising approach to retard the germination success of dinoflagellate cysts in constricted coastal areas, hence limiting the recurrence of harmful algal blooms in the water column.