Generation of reactive oxygen species by raphidophycean phytoplankton

Chattonella marina, a raphidophycean flagellate, is one of the most toxic red tide phytoplankton and causes severe damage to fish farming, Recent studies demonstrated that Chattonella sp, generates superoxide (O-2(-)), hydrogen peroxide (H2O2), and hydroxyl radicals (.OH), which may be responsible for toxicity of C. marina. In this study, we found that other raphidophycean flagellates such as Heterosigma akashiwo, Olisthodiscus luteus, and Fibrocapsa japonica also produce O-2(-) and H2O2 under normal growth condition. Among the flagellate species tested, Chattonella has the highest rates of production of O-2(-) and H2O2 as compared on the basis of cell number. This seems to be partly due to differences in their cell sizes, since Chattonella is larger than other flagellate species, The generation of O-2(-) by these flagellate species was also confirmed by a chemiluminescence assay by using 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (MCLA). All these raphidophycean flagellates inhibited the proliferation of a marine bacterium, Vibrio alginolyticus, in a flagellates/bacteria co-culture system, and their toxic effects were suppressed by the addition of superoxide dismutase (SOD) or catalase, Our results suggest that the generation of reactive oxygen species is a common feature of raphidophycean flagellates.