Effect of propionamide on the growth of Microcystis flos-aquae colonies and the underlying physiological mechanisms.

Reducing the formation and growth of Microcystis colonies is an important prerequisite for the effective prevention and treatment of cyanobacterial blooms. Microcystis flos-aquae colonies was selected to investigate the potential of propionamide for use in controlling cyanobacterial blooms. Propionamide, one of the major allelochemicals in the root exudates of E. crassipes, was tested using different concentrations (0, 0.2, 1, and 2mgL-1) and dosing methods (one-time addition, semi-continuous addition, and continuous addition) to assess its effect on the growth of M. flos-aquae colonies. The results showed that in the presence of different concentrations of propionamide, the growth of M. flos-aquae colonies followed a logistic growth model, with a higher degree of fit at lower propionamide concentrations. With the semi-continuous addition of 2mgL-1 propionamide, the growth of M. flos-aquae colonies was markedly inhibited; the relative inhibition ratio of algal cells reached >90% at day 7 of co-culture, and the colonial form gradually disintegrated, transforming mainly into unicellular and bicellular forms and small colonies (average diameter<50μm). Following the semi-continuous addition of 2mgL-1 propionamide, the exopolysaccharide content, the chlorophyll-a concentration, and the maximum photochemical efficiency of photosystem II (Fv/Fm) trended downward in M. flos-aquae colonies, whereas the relative expression of the microcystin (MC) biosynthetic genes, mcyA and mcyH, was upregulated overall. Importantly, the synthesis of intracellular microcystin-LR (MC-LR) was decreased after an initial increase, and the extracellular MC-LR concentration did not differ significantly from that in the control group (p>0.05). Moreover, an acute toxicity test showed that 2mgL-1 propionamide was generally non-toxic to Daphnia magna. In conclusion, appropriate use of propionamide could effectively control the expansion of M. flos-aquae colonies without potential risks to the ecological safety of aquatic environments; therefore, propionamide can actually be used to regulate cyanobacterial blooms in natural waters.