Dual Role of Cu2+ in the Physiological Growth and Metabolism of Microcystis aeruginosa

Cyanobacterial bloom is prevalent in freshwater of China and usually controlled by CuSO₄. However, the remaining available Cu²⁺ effects on cyanobacterial growth and metabolism have not been fully investigated. Therefore, we here investigated different CuSO₄ concentrations on the growth and metabolism of typical toxic cyanobacteria, Microcystis aeruginosa. The results showed that low CuSO₄ concentrations (<0.1 mg/L) facilitate algal growth, photosynthesis, and enzyme activity activities. Conversely, high CuSO₄ concentrations (>0.1 mg/L) inhibited the growth and markedly reduced the photosynthesis of oxidative enzymes. Notably, the algae secreted more extracellular polysaccharides (EPS) and released microcystin to mitigate the high Cu²⁺ stress compared to low concentrations. Moreover, the results illustrated how Cu²⁺ influenced the expression of key genes on the mRNA level. Specifically, capD (coded for polysaccharides), mcy (microcystin), and furA (mcy promoter) genes, were upregulated in high CuSO₄ concentrations, while the photosynthetic rbcL gene was downregulated. The furA regulated the mcy gene cluster promoter, which indirectly mediated the microcystin release. The upregulation of the complex genes revealed the intricate genetic responses of M. aeruginosa to Cu²⁺ stress. This study explored the dual roles of CuSO₄ in algae growth and molecular mechanisms, providing a new perspective on understanding the risks of algaecides application.