Oxygen Radical Production by a Red Tide Flagellate Chattonella and its Ichthyotoxicity

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Only when mortality occurred, hydrogen peroxide concentration ([H_2O_2]) increased up to ca. 1.5 mumol/l immediately after the onset of exposure of the yellowtail, Seriola quinqueradiata, to Chattonella. In cases where fish survived, H_2O_2 was not detected throughout the experiment. Fish showed no physiological perturbations in response to [H_2O_2] of 5 mumol/l added to ambient water. Thus, hydrogen peroxide per se was not responsible to fish kill by Chattonella. There was no detectable difference in [H_2O_2] between samples collected from buccal and opercular cavities. When the effect of mannitol, known as an OH radical scavenger, was tested, survival of fish was prolonged, suggesting the involvement of OH radical in fish kill mechanism by Chattonella.When the Japanese flounder, Paralichthys olivaceus, were exposed to Chattonella, blood Po_2 rapidly declined as was the case in the yellowtail. Unlike yellowtail, however, no fish was killed during 7 hr of exposure. The difference in su … More sceptibility to Chattonella between these two fish species was considered to be attributable to difference in hypoxia tolerance between them.We then compared the activities of catalase and superoxide dismutase (SOD) in the gills of the yellowtail, red sea bream, and flounder. The Vmax of catalase was significantly lower in the flounder than in the yellowtail, red sea bream showing an intermediate activity. The Km of catalase as well as activity of superoxide dismutase was not significantly different between the three species. This difference in catalase activity in the gills may also be responsible to the higher susceptibility of the yellowtail to Chattonella.Histological examination revealed that there were no edema formation in the gills when the initial Po_2 drop occurred early in Chattonella exposure. The only histological change observed in those gill samples was blockage of interfilamental spaces by mucus. Relationship between oxygen radical production by Chattonella and mucus secretion needs to be elucidated in the future research. Less