Synergistic and antagonistic interactions among five allelochemicals with antialgal effects on bloom-forming Microcystis aeruginosa

In this study, we investigated the allelopathic interactions among five representative allelochemicals at different proportions using bloom-forming Microcystis aeruginosa as the test receptor. Binary or ternary mixtures of allelochemicals obtained three types of allelopathic interactions, i.e., synergistic, antagonistic, and additive effects. However, combinations of four or five allelochemicals only yielded antagonistic effects. Interestingly, the algal inhibition gradually increased with the tested period for all treatments. For instance, the synergistic interaction occurred with the mixture including coumarin + ρ-hydroxybenzoic acid at 80%:20%; the mixture on the 10th day showed 0.80 of algal inhibition. While the mixture comprised protocatechuic acid + stearic acid + ρ-aminobenzene-sulfonic acid at 33.3%:33.3%:33.3%, the additive interactions occurred and then the maximum algal inhibition (0.83) was acquired at the end of tested period. In conclusion, the joint effects of different allelochemicals depend on various factors such as the chemicals used, their respective proportions, the total concentration of the mixture, and the receptor species. Thus, it is necessary to consider the complexity of allelopathic interactions and the field conditions during the control and management of noxious cyanobacteria.