Effects of bloom-forming species dinoflagellate Karenia mikimotoi on the development and photosynthetic characteristics of the sexually propagated embryos of macroalga Sargassum fusiformis

The sexual reproduction period (May–June) of Sargassum fusiformis, an important aquaculture seaweed in China, coincides with a time of high frequency of algal blooms caused by dinoflagellates such as Karenia mikimotoi. We studied the effects of dense K. mikimotoi suspensions on the sexual reproduction and subsequent development of the fertilized eggs of S. fusiformis in the laboratory. The results showed that K. mikimotoi did not inhibit the egg fertilization, but significantly (P < 0.05) inhibited the relative growth rate (RGR), chlorophyll (Chl) a content and maximum relative electron transport rate (rETRmax) of the embryos. On the fifth day of the culture period, the size of the embryos co-cultivated with K. mikimotoi at 1000 μmol Chl a L−1 began to decline significantly (P < 0.05) compared with the mono-cultured embryos and the difference increased with prolonged exposure. After 10 days, the RGR, Chl a content, and rETRmax of the embryos cultivated with K. mikimotoi decreased by 28.44%, 4.99%, and 63.99%, respectively, as compared with the ones cultivated without K. mikimotoi. These results indicated that the blooms formed by K. mikimotoi had little suppression on the fertilization of S. fusiforme eggs, but it did significantly inhibit the development and photosynthetic activities of the embryos, which would have adverse effects on the S. fusiforme farming industry.