Reactive oxygen species as causative agents in the ichthyotoxicity of the red tide dinoflagellate Cochlodinium polykrikoides

The underlying toxic mechanisms of the red tide dinoflagellate, Cochlodinium polykrikoides, were studied with respect to the reactive oxygen species-mediated toxic effect. Cochlodinium polykrikoides generates superoxide anion (O 2 -) and hydrogen peroxide (H 2 O 2 ), as measured by the cytochrome c reduction method and scopoletin-peroxidase method, respectively. The capability of C.polykrikoides to generate these oxygen radicals was related to the growth phase: the highest rate in the exponential phase and a gradual decrease in the stationary phase. Other phytoplankton, such as Eutreptiella gymnastica, Heterosigma akashiwo, Prorocentrum micans, Gymnodinium sanguineum and Alexandrium tamarense, also produce H 2 O 2 ; the rate of H 2 O 2 generation by these species was lower than that of C.polykrikoides. The exposure of liposomal samples to intact or ruptured individuals of C.polykrikoides resulted in severe membrane damage, such as liposomal lipid peroxidation. Cochlodinium polykrikoides-induced lipid peroxidation was significantly reduced by oxygen radical scavengers, superoxide dismutase, benzoquinone, catalase and mannitol. In addition, lipid peroxidation of gill tissue of flatfish exposed to C.polykrikoides increased with increasing algal cell density. These results suggest that reactive oxygen species generated from C.polykrikoides are responsible for oxidative damage leading to fish kills.