Harmful algal blooms are one of the greatest challenges when preserving water sources, especially when involving cyanobacteria such as Microcystis aeruginosa. Finding remediation possibilities is needed, and one of them has been the use of macrophytes such as the species Myriophyllum, which have presented allelopathic mechanisms of phytoplankton control. Thus, this work aimed to evaluate the inhibition of M. aeruginosa cell growth in a co-exposure with Myriophyllum aquaticum and the influence on microcystin-LR concentration. The experiments were carried out using a culture of M. aeruginosa (1x106 cells mL-1) in a co-exposure with M. aquaticum for seven days. The inhibitory effects were investigated by counting the cells; the effects on photosynthetic pigments were measured and microcystin-LR was quantified in the culture medium on the last experimental day. To evaluate the possible effects of competition for nutrients and space, the concentration of total orthophosphate was quantified and treatment with plastic plants was used. The experiments with Myriophyllum aquaticum achieved the total inhibition of M. aeruginosa growth and a significant reduction of the photosynthetic pigments (> 98%). Additionally, we observed a reduction of microcystin-LR concentration (79%) in the tests with macrophytes when compared to the control. Competition for space and nutrients was not observed, demonstrating that the effects on M. aeruginosa were caused by aquatic macrophyte presence. These results may indicate that M. aquaticum causes inhibitory effects on cyanobacteria growth by allelopathic effects and removes microcystin-LR.

Myriophyllum aquaticum has shown potential allelopathic effects for the biocontrol of cyanobacteria and cyanotoxins. However, the composition of allelochemicals and their biological effects may be influenced by seasonal changes. In this study, we investigated the impact of aqueous extracts of M. aquaticum collected in different seasons on the growth of Microcystis aeruginosa and the concentration of microcystin-LR. Plant samples were extracted using ultrasound cycles in aqueous solutions, and extracts at varying concentrations (0.1, 10, and 100 mg/L) and a control treatment were inoculated with M. aeruginosa, and cell growth was analyzed using a Neubauer chamber. Photosynthetic pigment quantification was used to measure physiological effects and liquid chromatography was used to evaluate the microcystin-LR concentrations. The extracts of plants collected during autumn and winter exhibited higher inhibition of M. aeruginosa growth and a reduction in photosynthetic pigments compared to those collected during spring and summer. These results can be explained by the higher presence of phenolic compounds in the composition of extracts from autumn and winter. Microcystin-LR concentrations were decreased at 10 and 100 mg/L, with the highest efficiency observed in autumn, while spring showed lower efficiency. Our findings suggest that M. aquaticum extracts have inhibitory potential on M. aeruginosa, particularly during the autumn season, making them a promising nature-based solution for the biocontrol of harmful algal blooms.