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