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A novel single-stranded RNA (ssRNA) virus specifically infecting the bloom-forming diatom Rhizosolenia setigera (R. setigera RNA virus [RsRNAV]) was isolated from Ariake Sea, Japan. Viral replication occurred within the cytoplasm, and the virus particle was icosahedral, lacked a tail, and was 32 nm in diameter on average. The major nucleic acid extracted from the RsRNAV particles was an ssRNA molecule 11.2 kb in length, although smaller RNA molecules (0.6, 1.2, and 1.5 kb) were occasionally observed. The major structural proteins of RsRNAV were 41.5, 41.0, and 29.5 kDa. Inter-and intraspecies host specificity tests revealed that RsRNAV is not only species specific but also strain specific and that its intraspecies host specificity is diverse among virus clones. The latent period of RsRNAV was 2 days, and the burst sizes were 3,100 and 1,010 viruses per host cell when viruses were inoculated into the host culture at the exponential and stationary growth phases, respectively, at 15°C under a 12-h-12-h light-dark cycle of ca. 110 mol of photons m ؊2 s ؊1 with cool white fluorescent illumination. To our knowledge, this is the first report describing the biological properties of a virus infecting a diatom. Further studies on RsRNAV will be helpful in understanding the ecological relationship between diatoms and viruses in nature.

The growth characteristics and intraspecies host specificity of Heterocapsa circularisquama virus (HcV), a large icosahedral virus specifically infecting the bivalve-killing dinoflagellate H. circularisquama, were examined. Exponentially growing host cells were more sensitive to HcV than those in the stationary phase, and host cells were more susceptible to HcV infection in the culture when a higher percent of the culture was replaced with fresh medium each day, suggesting an intimate relationship between virus sensitivity and the physiological condition of the host cells. HcV was infective over a wide range of temperatures, 15 to 30°C, and the latent period and burst size were estimated at 40 to 56 h and 1,800 to 2,440 infective particles, respectively. Transmission electron microscopy revealed that capsid formation began within 16 h postinfection, and mature virus particles appeared within 24 h postinfection at 20°C. Compared to Heterosigma akashiwo virus, HcV was more widely infectious to H. circularisquama strains that had been independently isolated in the western part of Japan, and only 5.3% of the host-virus combinations (53 host and 10 viral strains) showed resistance to viral infection. The present results are helpful in understanding the ecology of algal host-virus systems in nature.

To examine the relationship between the bloom-forming dinoflagellate Heterocapsa circularisquama and its infectious viruses, a field survey was conducted in Ago Bay, Japan, in 2001. A H. circularisquama bloom occurred in July. The bloom peaked in mid July and disintegrated within a few days at the end of July. The abundance of viruses infectious to H. circularisquama was high from the peak of the bloom and throughout the post-bloom period, but ceased by the end of August. At the peak of the bloom, 88% of the H. circularisquama cells in the population harbored small viruslike particles (VLPs). Based on transmission electron microscopic (TEM) observation, morphological resemblance between these VLPs and the single-stranded RNA (ssRNA) virus infecting H. circularisquama (HcRNAV: H. circularisquama RNA virus) isolated from the bloom was noticeable. The fluctuation patterns of the viruses indicated that at least 2 distinct types of virus with different host specificity spectra coexisted. A specific increase in viral abundance in the sediments was observed in the middle of the bloom, and these viruses were likely able to maintain their infectivity for at least 3 mo. The present study provides further evidence of the possible viral impacts on the biomass and clonal composition of algal populations in the natural environment, and offers support for the hypothesis that sediments are a reservoir of algal viruses.

ABSTRACT. A total of 233 marine bacterial stra~ns which killed a noxious marine microalga, Heterosigma akashiwo, were isolated from Hiroshima Bay, the Seto Inland Sea, Japan, during blooms of H. akashirvo in 1994 and 1995. Population structure and genetic diversity of the H. akashiwo-killing bacteria (HAKB) were analyzed by means of restriction fragment length polymorphism (RFLP) and partial sequences of 16s nbosomal RNA genes (16s rDNA) PCR (polymerase chain reaction) amplified from HAKB strains. The RFLPs were generated by separate digestion with 5 restriction enzymes, Eco RI, Rsa I , Mbo I, Bst U1 and Hha I. Seventeen ribotypes were observed among 85 strains of HAKB isolated in 1994. Bacterial strains of 3 ribotypes, 2B, 2C and 2D, were dominant in the HAKB populations during the termination period of the H akashlwo bloom concur.rent with the increase in the number of HAKB. Partial sequences, almost 500 bp of nucleotides, and RFLP patterns of 16s rDNA from some HAKB strains revealed that the HAKB of 2B, 2C and 2D ribotypes are closely related to the 'I-proteobacteria group. The HAKB strains belonging to 2C and 2D were repeatedly isolated from seawater collected at the end of a H. akashjwo bloom in 1995. These results suggest that 3 species of HAKB may play a role in the rapid termination of the H. akashiwo bloom in Hiroshima Bay.