Inhibitory mechanism of Macleaya cordata crude extracts on freshwater cyanobacterium Microcystis aeruginosa.

A promising solution to combat harmful cyanobacterial blooms (CyanoHABs) is the use of naturally-derived cyanocidal compound. This study found that the crude extracts of the herbaceous plant Macleaya cordata can inhibit the growth of the bloom-forming cyanobacterium Microcystis aeruginosa. Growth inhibition test suggested that the inhibitory effect was significant at a concentration as low as 5 µg/L of the crude extracts. To comprehensively elucidate inhibitory mechanisms, we examined the responses in anti-oxidative systems, changes in triggering signals in cells, and hallmarks of programmed cell death (PCD) under the two levels of exposure to the crude extracts of M. cordata. A notable observation across all treatment groups was the significant increase in the superoxide dismutase activity of M. aeruginosa at 72 h. However, distinct patterns were observed in malondialdehyde (MDA), with no significant difference observed between the low concentration treatment group (5 µg/L) and the control group. In contrast, Microcystis cells subjected to high concentration (10 µg/L) exhibited a significant difference in MDA content at both 24 and 72 h. Furthermore, we observed that the levels of potential signaling molecules, e.g., intracellular reactive oxygen species, nitric oxide, and Ca were altered in the Microcystis cells after exposure to the crude extracts. This results in alterations to the levels of signaling molecules, which in turn stimulate the upregulation of orthocaspase, leading to PCD and population collapse eventually. This study examines the mechanisms of Microcystis suppression through naturally-derived substances, providing theoretical and technical foundations for the sustainable management of CyanoHABs.