In recent years macrophytes have been considered promising tools in the biocontrol of harmful cyanobacteria blooms (cyanoHABs). In this study, the inhibitory effect of aqueous extracts of Ranunculus aquatilis and Nasturtium officinale on Microcystis aeruginosa growth was assessed via six treatments (0%, 0.1%, 0.25%, 0.50%, 0.75%, and 1% extracts). Chlorophyll a and carotenoid content were analyzed and changes in cell and colonial morphology of M. aeruginosa cultures were observed. Also, to reveal potential allelochemical compounds, total phenols (TPs), total flavonoids (TFs), and tannins (TTs) were analyzed in both extracts. The obtained results showed that M. aeruginosa growth was significantly inhibited by R. aquatilis and N. officinale aqueous extracts in a concentration-dependent way. After 8 days of treatment, the highest inhibition rates reached 100% and 75.74% respectively. The Chlorophyll a and carotenoid concentrations were decreased compared to the control group. Colonial and cell and colonial morphology changes were observed under the treatment group with 1% of aqueous extract accompanied by sedimentation of the cyanobacterial cells. This study shows that M. aeruginosa growth inhibition was induced by the total polyphenol, flavonoids and tannins. It was concluded that theses macrophytes may control M. aeruginosa and may be useful to control harmful blooms in lake-reservoirs.

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Abstract. In search of an ecofriendly algaecide, aqueous extracts of two medicinal plants, Thymus satureioides Coss. and Artemisia herba alba L., were assessed for antialgal activity against Microcystis aeruginosa. An experiment was designed using five treatments (1%, 0.75%, 0.5%,0.25% and 0.1%). The growth of M. aeruginosa, morphological modifications, and photosynthetic pigments (chlorophyll a and carotenoids) on exposure to the extracts were explored. Also, phytochemical parameters in the extracts were analyzed to reveal the potential allelochemical compounds.The results showed that both T. satureioides and A. herba alba extracts inhibited the growth of M. aeruginosa in a concentration-dependent way. After 8 days of treatment, the highest inhibition rates reached were 95%, 93% and 88.58%, and for T. satureioides and A. herba alba aqueous extracts, respectively. Chlorophyll a and carotenoid concentrations in cultures decreased especially in the 1% treatment group. Several morphological changes were observed in the treatment group compared to the controls. It was concluded that M. aeruginosa growth was suppressed by the potentially allelochemical compounds and probably by other allelochemical substances in aqueous extracts. Our results illustrated that both T. satureioides and A. herba alba extracts are able to control Microcystis blooms, and these may be recommended as a remedy for contamination of water bodies by harmful blooms.

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