Nagasaki K, Tarutani K, Itakura S & Yamaguchi M
Red Tide Research Division, Nansei National Fisheries Research Institute,
Ohno, Saeki, Hiroshima 739-0452, Japan
Heterosigma akashiwo (Raphidophyceae) is one of the typical HAB (harmful algal bloom)-causing phytoplankton, which is distributed in coastal waters of subarctic and temperate areas of both northern and southern hemispheres. In 1993, it was found that the proportion of H. akashiwo cells containing VLPs (virus-like particles) specifically increased in the final stage of a red tide. This finding suggested that there could be some kind of relationship between the appearance of VLPs and the disintegration of a H. akashiwo red tide. We have succeeded in isolating 15 clones of a virus infecting and killing H. akashiwo (HaV: Heterosigma akashiwo virus) in 1996. Their morphological feature is highly comparable to that of the VLP observed in the natural red tide samples. Since the infectivity of HaV is highly specific to H. akashiwo, HaV has the potential to disintegrate H. akashiwo red tides. However, high diversity among both virus clones and host strains regarding viral infection (intra-species host specificity) was found. This result indicates that the interaction between viruses and hosts in situ appears more complicated than a simple host and pathogen relationship. HaV is quite unstable under simulated in situ environmenta conditions (light and temperature). Thus, it is unclear how HaV survives, i.e. maintains its infectivity in the natural environment. These problems have to be solved in order to elucidate the role of HaV in the regulation of H. akashiwo red tides.