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 thereactive oxygen species-mediated toxic effect. Cochlodiniumpolykrikoides generates superoxide anion (O₂^–) and hydrogenperoxide (H₂O₂), as measured by the cytochrome c reduction methodand scopoletin–peroxidase method, respectively. The capabilityof C.polykrikoides to generate these oxygen radicals was relatedto the growth phase: the highest rate in the exponential phaseand a gradual decrease in the stationary phase. Other phytoplankton,such as Eutreptiella gymnastica, Heterosigma akashiwo, Prorocentrummicans, Gymnodinium sanguineum and Alexandrium tamarense, alsoproduce H₂O₂; the rate of H₂O₂ generation by these species waslower than that of C.polykrikoides. The exposure of liposomalsamples to intact or ruptured individuals of C.polykrikoidesresulted in severe membrane damage, such as liposomal lipidperoxidation. Cochlodinium polykrikoides-induced lipid peroxidationwas significantly reduced by oxygen radical scavengers, superoxidedismutase, benzoquinone, catalase and mannitol. In addition,lipid peroxidation of gill tissue of flatfish exposed to C.polykrikoidesincreased with increasing algal cell density. These resultssuggest that reactive oxygen species generated from C.polykrikoidesare responsible for oxidative damage leading to fish kills.