Hormesis and Antagonism in Low-Dose <i>Phalaris</i> Allelochemicals during <i>Microcystis</i> Control

Aquatic ecosystems face significant challenges globally from cyanobacterial blooms. <i>Phalaris arundinacea</i> (Reed Canary Grass) is used in artificial wetlands and found in natural wetlands. We investigated whether allelochemicals released from <i>Phalaris</i> root exudates inhibit <i>Microcystis aeruginosa</i> 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 <i>Microcystis</i> 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 <i>Microcystis,</i> suggesting that protists and rotifers may control <i>Microcystis</i> growth. We also tested commercial allelochemicals at environmentally relevant concentrations (≤ 1000 µg Lˉ¹) against <i>Microcystis</i>. 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 <i>Microcystis</i> (≥ 87%). The minimum effective concentration was determined to be 100 µg Lˉ¹ for most allelochemicals, except for anthraquinone, which had a hormetic effect stimulating <i>Microcystis</i> growth by up to 70% compared to the controls. Our findings indicate that allelochemicals could be used to control <i>Microcystis,</i> but it is essential to establish the effective allelochemical minimum inhibitory concentrations from biofilters, wetlands, or macrophytes to assess their potential for managing <i>Microcystis</i>, 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.