Hormesis and Antagonism in Low-Dose Phalaris Allelochemicals during Microcystis Control

Aquatic ecosystems face significant challenges globally from cyanobacterial blooms. Phalaris arundinacea (Reed Canary Grass) is used in artificial wetlands and found in natural wetlands. We investigated whether allelochemicals released from Phalaris root exudates inhibit Microcystis aeruginosa growth. We conducted experiments to disentangle the effect of the root exudates from living plants on resource competition and the potential role of microbiota in controlling Microcystis growth. We found that allelochemicals from root exudates and their inhibitory effect decayed over time. Results from filtration experiments and microscopic observations indicated that the removal of microorganisms (≥ 0.22 µm) allowed for the growth of Microcystis, suggesting that protists and rotifers may control Microcystis growth. We also tested commercial allelochemicals at environmentally relevant concentrations (≤ 1000 µg Lˉ¹) against Microcystis. Concentrations of 1000 µg Lˉ¹ of anthraquinone, gallic acid, gramine, hordenine, linoleic acid, naringuin, stigmasterol, tannic acid, 4-nitroindol-5-carboxaldehyde and the mixture of the nine allelochemicals inhibit Microcystis (≥ 87%). The minimum effective concentration was determined to be 100 µg Lˉ¹ for most allelochemicals, except for anthraquinone, which had a hormetic effect stimulating Microcystis growth by up to 70% compared to the controls. Our findings indicate that allelochemicals could be used to control Microcystis, but it is essential to establish the effective allelochemical minimum inhibitory concentrations from biofilters, wetlands, or macrophytes to assess their potential for managing Microcystis, other cyanobacteria, and microalgae. The increasing global use of artificial wetlands to control cyanobacterial blooms justifies further investigation into allelochemical concentrations, including decaying trends over time and hormetic effects.