The effect of saxitoxin and non-saxitoxin extracts of Cylindrospermopsis raciborskii (Cyanobacteria) on cyanobacteria and green microalgae

The effect of saxitoxins (STX) on phytoplankton species is poorly understood. To date, no correlation between STX concentrations and phytoplankton physiology has been reported. We investigated the effect of STX (STX+, 0.5–10 μg L total STX) and non-STX (STX−, 0.5–10 μg L total STX biomass equivalent) extracts of Cylindrospermopsis raciborskii on Microcystis wesenbergii BCCUSP11, Microcystis aeruginosa BCCUSP232 (microcystin producing), Scenedesmus acuminatus UFSCar036, and Monoraphidium convolutum CMEA/UFF0201 under controlled laboratory conditions. Both STX+ and STX− extracts inhibited the cell density and specific growth rate of M. wesenbergii, M. aeruginosa, and S. acuminatus. However, the effect of STX+ extract on the phytoplankton strains was significantly higher than that of STX− extract. M. convolutum, on the other hand, was tolerant as both STX+ and STX− extracts did not significantly reduce its cell density and specific growth rate (day). The exposure of M. aeruginosa to STX+ and STX− resulted in higher total (intracellular and extracellular) microcystin concentration than the control. STX concentrations had a significant negative correlation with cell density and growth response of the phytoplankton strains investigated in this study. Conclusions can be made that although both STX+ and STX− extracts of C. raciborskii inhibited the growth of some phytoplankton species, the STX+ extracts were more toxic.