Growth interactions between the dinoflagellate Karenia selliformis and the diatom Chaetoceros diadema

.The harmful algal bloom species Karenia selliformis caused massive fisheries damage in the waters off eastern Hokkaido, Japan, in September and October 2021. The occurrence of red tides is frequently linked to the complex interactions among various phytoplankton species. It is therefore important to elucidate the mechanisms of bloom formation and species interactions between K. selliformis and other algae, especially diatoms. Diatom cells almost disappeared during this bloom, suggesting that competitive growth is involved in the formation of the K. selliformis-dominated bloom. To this end, we investigated growth competition and possible allelopathy between K. selliformis and the diatom Chaetoceros diadema isolated from seawater and sediment, collected during the red tide outbreak in Hokkaido, Japan. In the mixed-culture experiments, species with higher initial cell density (2000 cells ml−1) suppressed growth in the other species with lower initial cell density (200 cells ml−1). In experiments with the same initial cell densities, K. selliformis always outcompeted C.diadema, showing that K. selliformis has a more pronounced inhibitory effect on C. diadema. Numerical analyses using the Lotka–Volterra model supported these observations. We also observed an allelopathic effect of K. selliformis on C. diadema in the inhibitory effect of culture filtrate; cell contact enhanced the growth suppression. These results suggest that the K. selliformis-dominated bloom near Hokkaido was sustained by high cell densities of K. selliformis depriving the diatoms of an opportunity to grow.