Interactions between the seaweed Gracilaria and dinoflagellate Akashiwo sanguinea in an indoor co-cultivation system and the interference of bacteria

Many studies have demonstrated that some macroalgae (including the widely cultivated red alga Gracilaria lemaneiformis) can inhibit the growth of microalgae via allelopathy and resource competition, with the former as the major contributor. However, little is known currently about whether or not and how microalgae react as a feedback to the inhibitory effects or, more generally, the interactions between seaweed and microalgae in their co-culturing system. Here, we report a laboratory-based study on the interactions between the seaweed Gracilaria lemaneiformis and the common harmful algal blooms (HABs)-forming dinoflagellate Akashiwo sanguinea. We found that while both the fresh thalli of G. lemaneiformis and the extracts of fresh and dried G. lemaneiformis could significantly inhibit the growth of A. sanguinea, the dead cells of A. sanguinea “revenged” G. lemaneiformis via promoting the growth of bacteria and consequently slowed the growth of G. lemaneiformis, which was evidenced by the lowered pH, slowed nutrients consumption in the cultures, the elevated counts of bacteria, and the reduced biomass of G. lemaneiformis. Collectively, our results showed that while G. lemaneiformis could inhibit the growth of microalgae with allelopathy as a major contributor, the death of allelopathy-affected microalgae could promote bacterial growth, which sequentially inhibits the growth of G. lemaneiformis as a feedback.