Competitive interactions between two allelopathic algal species: Heterosigma akashiwo and Phaeodactylum tricornutum

Both Heterosigma akashiwo and Phaeodactylum tricornutum have been reported to produce allelochemicals capable of inhibiting the growth of co-occurring microalgae. Here, potential allelopathy between H. akashiwo and P. tricornutum was evaluated using bi-algal culture, cell-free culture filtrate and a no-contact co-culturing system in nutrient-replete media. Experiments were also conducted in the no-contact co-culturing system under nutrient-limited conditions. In nutrient-replete bi-algal culture, the growth of P. tricornutum and H. akashiwo each tended to be strongly suppressed when the other species was inoculated at high cell densities. A mathematical model was used to simulate the growth interactions of the two species in bi-algal culture and showed that P. tricornutum outcompeted H. akashiwo over time with different initial cell densities under nutrient-sufficient conditions, indicating that P. tricornutum was more potent in allelopathy. Heterosigma akashiwo growth was inhibited both in the P. tricornutum culture filtrate and no-contact co-culturing system. This confirmed that the extracellular allelopathic compounds released by P. tricornutum were one of the sources of the H. akashiwo growth inhibition. Nutrient deficiency did not increase the extent of allelopathic activity of allelochemicals.