Hemolytic Activity in Relation to the Photosynthetic System in and .

species, and , are harmful raphidophycean flagellates known to have hemolytic effects on many marine organisms and resulting in massive ecological damage worldwide. However, knowledge of the toxigenic mechanism of these ichthyotoxic flagellates is still limited. Light was reported to be responsible for the hemolytic activity (HA) of species. Therefore, the response of photoprotective, photosynthetic accessory pigments, the photosystem II (PSII) electron transport chain, as well as HA were investigated in non-axenic and cultures under variable environmental conditions (light, iron and addition of photosynthetic inhibitors). HA and hydrogen peroxide (HO) were quantified using erythrocytes and pHPA assay. Results confirmed that% HA of was initiated by light, but was not always elicited during cell division. Exponential growth of and under the light over 100 µmol m s or iron-sufficient conditions elicited high hemolytic activity. Inhibitors of PSII reduced the HA of , but had no effect on . The toxicological response indicated that HA in was not associated with the photoprotective system, i.e., xanthophyll cycle and regulation of reactive oxygen species, nor the PSII electron transport chain, but most likely occurred during energy transport through the light-harvesting antenna pigments. A positive, highly significant relationship between HA and chlorophyll (chl) biosynthesis pigments, especially chl and chl , in both species, indicated that hemolytic toxin may be generated during electron/energy transfer through the chl biosynthesis pathway.