In this study, a water-soluble metal-free phthalocyanine (SPC) containing sodium 2-mercaptoethanesulfonate substituents at the peripheral positions was used to investigate the algaecidal properties and oxidative effects on the growth of two microalgal species, Arthrospira platensis and Chlorella vulgaris . Although OD at 560 nm and chlorophyll- a content were decreased in Arthrospira platensis during 7 days depending on dose and time, increases in both OD at 750 and chlorophyll- a content at 8 ppb (parts per billion) concentration on the 7th day were observed in Chlorella vulgaris . However, total SOD (superoxide dismutase) and GR (glutathione reductase) enzyme activity of A. platensis cultures did not display any alteration in all concentrations, SOD activity displayed an increase significantly at 2 ppb concentration, and GR activity showed increases at 1, 2, and 4 ppb concentrations in C. vulgaris application. In A. platensis application, APX (ascorbate peroxidase) activity decreased at 0.50 ppb, 1 ppb, and 1.5 ppb concentrations. In addition, C. vulgaris application showed decreases at all concentrations. When MDA content increased at all concentrations, the H 2 O 2 content increased only at significatly 0.125 ppb concentration in A. platensis cultures. Both MDA (malondialdehyde) and H 2 O 2 (hydrogen peroxide) content of C. vulgaris cultures showed a statistically significant decrease at all concentrations compared to control. Free proline decreased at 0.25 ppb, 0.50 ppb, 1 ppb, and 1.5 ppb concentrations in A. platensis application, and it decreased at all the concentrations of C. vulgaris application. It concluded that this compound has inhibition effects on A. platensis , but it supports growth in C. vulgaris . Therefore, this synthesized phthalocyanine compound (SPC) should be consumed carefully, and the contamination to aquatic ecosystems should be prevented.