Toxic potential of the raphidophyte Olisthodiscus luteus: mediation by reactive oxygen species

Olisthodiscus sp. is classified in the class Raphidophycea of the division Chrysophyta. Other raphidophytes, Heterosigma spp. and Chattonella spp., ar e well known as the causative organisms of red tides, and are highly toxic to fish. However, the toxic potential of Olisthodiscus spp. has not been es tablished. Recent studies demonstrated that Heterasigma spp. and Chattonell a spp. generate reactive oxygen species (ROS) such as O-2(-) and H2O2 In th is study, we found that Olisthodiscus luteus also produces O-2(-) and H2O2 under normal growth conditions. The levels of O-2(-) and H2O2 produced by O .luteus were comparable to those of Heterosigma akashiwo. We therefore spec ulated that O.luteus also exerts ROS-mediated harmful effects on surroundin g living organisms. In fact, O.luteus inhibited the proliferation of marine bacteria, Vibrio alginolyticus, in a plankton/bacteria co-culture system. Furthermore, the toxic effect of O.luteus on V.alginolyticus was suppressed by the addition of superoxide dismutase (SOD) and catalase. Although the d etailed mechanism of the production of ROS by O.luteus is still unclear, O- 2(-) production was increased by treatment with lectins such as wheat germ agglutinin (WGA), castor bean hemagglutinin (CBH) and concanavalin A. (Con A), suggesting that cell surface redox enzymes may be responsible for the p roduction of ROS. This hypothesis was supported by the finding that protein ase K inhibited O-2(-) generation by O.leteus. In conclusion, our results s uggest that O.luteus exhibits ROS-mediated harmful effects on surrounding l iving organisms.