Isolation of a virus infectious to the harmful bloom causing microalga Heterosigma akashiwo (Raphidophyceae)

Abstract

A virus infecting the harmful bloom causing microalga Heterosigma akashiwo (Raphidophyceae) was isolated from the coastal water of Nomi Bay, Japan, in July 1996. The isolate caused lysis in 2 strains of H. akashiwo tested and numerous virus-like particles (VLPs) appeared in the lysed algal culture, whereas virus multiplication was not detected in the healthy culture of H. akashiwo without its inoculation. Thus, fulfilling Koch's postulate, it was considered to be a virus and designated HaV (Heterosigma akashiwo virus) clone GSNOU-30. The virus particle is icosahedral, lacking a tail, and 202 ± 6 nm (average ± standard deviation) in diameter with an electron-dense roundish core that is distinct from the capsid. The virus stained positive with DAPI, indicating that it possesses a double stranded DNA genome. The virus proliferated in the protoplasm of the host cell as had previously been observed in H. akashiwo cells from a natural red tide population. The virus did not cause lysis of Chattonella antiqua, C. verruculosa or Fibrocapsa japonica (Raphidophyceae) as well as 15 strains of phytoplankton belonging to other classes. It is most noteworthy that 3 strains of H. akashiwo isolated from Hiroshima Bay, Japan, were resistant to GSNOU-30, suggesting that the viral infectivity is not species-specific but strain-specific. These results suggest that the virus is involved in the population dynamics of H. akashiwo, playing a role as a selector to increase genetic diversity of a host species.