VIRAL MORTALITY IN THE FINAL STAGE OF HETEROSIGMA-AKASHIWO (RAPHIDOPHYCEAE) RED TIDE

During a bloom of Heterosigma akashiwo (Raphidophyceae) in northern Hiroshima Bay, Japan, in June 1993, the proportion of H. akashiwo cells containing virus-like particles (VLPs) was monitored by transmission electron microscopy. Until 3 days before the termination of the red tide, no VLP-containing cells were detected, and the proportion of VLP-containing cells was < 1% on the last 2 days of the red tide. However, the sample collected on the last day, continuously incubated for 26 h at 22-degrees-C, revealed a high frequency (11.5%) of VLP-containing cells. These findings suggest that viral mortality occurs in quite a short time and may play an important role in regulating the disintegration of H. akashiwo red tide.