Palmatine chloride inhibits Microcystis aeruginosa via photosynthetic impairment, oxidative stress, and altered cellular energy status.

Harmful cyanobacterial blooms threaten aquatic ecosystems and public health through cyanotoxin release and oxygen depletion. This study evaluated the inhibitory activity of palmatine chloride (PC) against Microcystis aeruginosa under different light and temperature conditions and investigated the associated physiological, biochemical, ultrastructural, and transcriptional responses. At 25°C, PC maintained concentration-dependent inhibition across 50-125 μmol·m-2·s-1, achieving 48-52%, 92-96%, and 90-98% inhibition at 4, 8, and 12 mg/L, respectively, within seven days. At 35°C, 12 mg/L PC produced near-complete inhibition within 48 h. Specifically, PC markedly impaired photosynthetic performance, increased reactive oxygen species and malondialdehyde levels, altered antioxidant responses, and induced membrane disruption, thylakoid disorganization, and cytoplasmic leakage. N-acetylcysteine co-treatment did not restore growth or photosynthetic parameters, indicating that PC inhibition was not solely attributable to an NAC-reversible ROS-dependent process. Transcriptomic profiling and ATP measurements further indicated substantial disruption of cellular energy homeostasis, characterized by downregulation of genes involved in photosynthesis, respiration, and central carbon metabolism, together with an 84% decrease in intracellular ATP. Preliminary toxicity tests suggested relatively low acute toxicity to Daphnia magna and Vibrio fischeri under the tested conditions. Besides, PC had limited effects on Spirulina platensis but modestly inhibited Chlorella vulgaris, while no sustained accumulation of extracellular microcystin equivalents was detected. Collectively, PC strongly inhibited Microcystis aeruginosa, accompanied by photosynthetic impairment, oxidative stress, structural damage, and disruption of cellular energy homeostasis. These findings support further evaluation of PC as a candidate botanical algicide.