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14 clones of the Heteroslgma akashiwo virus (HaV) infecting the harmful bloom causlng mcroalga Heteroslgrna akashiwo (Raphidophyceae) u r r e isolated from coastal waters of Japan Host specifirlt~es ot the virus clones were tested against 18 stralns of H dkashiwo isolated from coastal waters of the western part of Japan Although all of the 14 virus clones lysed 10 of the H akash~rvo stralns. in 4 stralns no viral lysis was observed with any of the clones The remaining 4 H akashiwro stralns were lysed by 7 to 11 of the virus clones Further viral l y s ~ s against H akash~wo 1s not always complete, al lolv~ng the prolifelation of surviving cells These results i nd~ca te 2 poss~bihties (1) HaVs are phenotypically dlvelse In terms of t h e ~ r host speclfic~ty (2) H akash~wo strains are phenotypically diverse In terms of thelr vlral sensitivity The high diversity among both the vlrus clones and the host stralns concerning v ~ r a l ~ n f e c t ~ o n indicates that the interaction between viruses and hosts in s ~ t u appears more complicated than a simple host and pathogen re la t~on KEY WORDSHaV . Heteroslgma akashi~vo . Raphidophyceae . Harmful algal bloom . Red tide . Virus . Host range In recent years, viruses infecting 5 different eukaryotic marine microalgae have been isolated: Micl-onlonas pusilla (Prasinophyceae), Aureococcus anophagefferens (Chrysophyceae), Chrysochromulina spp. (Prymnesiophyceae), Phaeocystis pouchetii (Prymnesiophyceae) and Heterosigma akashiwo (Raphidophyceae) (Cottrell & Suttle 1991, Milligan & Cosper 1994, Suttle & Chan 1995, Jacobsen et al. 1996, Nagasaki & Yamaguchi 1997). Their host range appears to be quite narrow as they lyse only the host species used in each screening experiment with the exception of a virus infectious to both C. brevifilum and C. strobilus (Suttle & Chan 1995). However, there have been few 'E-mail: nagasaki@nnf.affrc go.]p O Inter-Research 1998 surveys of any intra-species host specificity, i.e. the strain specificity of these viruses. We have recently reported on the strain specificity of the Heterosigma akashiwo virus (HaV) clone GSNOU30, designated as 'HaV08' in the present paper (see Table 1). In the previous study, this clone was lytic to only 2 of 5 s t r a~ns of H. akashiwo tested, suggesting marked phenotypic diversity among H. akashiwo strains (Nagasaki & Yamaguchi 1997). In the present note, we describe the lytic activity of 14 clones of HaV against 18 clonal strains of H. akashiwo isolated from coastal waters in western Japan to examlne the diversity of both virus clones and algal strains in terms of infectivity and susceptibility. Seawater samples collected in Unoshima Port (Fukuoka Prefecture) on 24 June 1996 and Nomi Bay (Kochi Prefecture) on 11 July 1996, which contained 17 000 and 35400 cells ml-' of Heterosigma akashiwo, respectively, were initially kept at 4'C and sent to the laboratory within 24 h . They were UV treated as described previously (Nagasaki & Yamaguchi 1997) and screened for the presence of viruses lytic to H. akashiwo strains UR94, GS95, KG95, KM95 or H93616. The seawater sample collected in northern Hiroshima Bay (Hiroshima Prefecture) on 3 July 1996, containing 700 cells ml-' of H. akashiwo, was concentrated 127-fold using a Hollow Fiber Cartridge system (1 MW cutoff, AMICON) and filtered through a 0.2 pm Nuclepore membrane filter. The filtrate was also screened for the presence of algicidal factors against the 5 strains of H. akashiwo, and they were isolated by performing the extinction dilution method twice (Nagasaki & Yamaguchi 1997). The MPN (most probable number) of algicidal units in each extinction dilution assay was calculated using the computer program developed by Nishihara et al. (1986). MPN of 110 Aquat Microb Ecol 14: 109-112, 1998 Table 1. Heterosigma akashiwo. Origin of virus clones isoIn the present screening experiment, 32 algicidal lated in the present study. Host strain: H. akashjwo strain factors were isolated. considering their morphological used as a host in the screening experiment; irradiation timeexposure time to UV irradiation (see Nagasaki & Yarnaguch~ features, DAPI stainability and fulfillment of Koch's 1997) postulate, they are concluded to be viruses infectious

Intra-species host specificity of HaV (Heterosigma akashiwo virus) clones