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.

This is the first report to explore the fine-scale diversity, population genetic structure, and biogeography of a typical planktonic microbe in Japanese and Korean coastal waters and also to try to detect the impact of natural and human-assisted dispersals on the genetic structure and gene flow in a toxic dinoflagellate species. Here we present the genetic analysis of Alexandrium tamarense (Lebour) Balech populations from 10 sites along the Japanese and Korean coasts. We used nine microsatellite loci, which varied widely in number of alleles and gene diversity across populations. The analysis revealed that Nei's genetic distance correlated significantly with geographic distance in pair-wise comparisons, and that there was genetic differentiation in about half of 45 pair-wise populations. These results clearly indicate genetic isolation among populations according to geographic distance and restricted gene flow via natural dispersal through tidal currents among the populations. On the other hand, high P-values in Fisher's combined test were detected in five pair-wise populations, suggesting similar genetic structure and a close genetic relationship between the populations. These findings suggest that the genetic structure of Japanese A. tamarense populations has been disturbed, possibly by human-assisted dispersal, which has resulted in gene flow between geographically separated populations. 1

A virus infecting the novel shellfish-killing dinoflagellate Heterocapsa circularisquama (H. circularisquama Virus: HcV) was isolated from Japanese coastal waters in August 1999 during a H. circularisquama bloom. Transmission electron microscopy of ultrathin sections of infected H. circularisquama revealed the presence of intracellular virus-like particles 24 to 48 h after infection. The virus was icosahedral, lacking a tail, ca 180 to 210 nm (mean ± standard deviation = 197 ± 8 nm) in diameter and contained an electron-dense core. It was a double-stranded DNA virus, and the appearance of the virus particles was associated with a granular region (viroplasm) in the cytoplasm that did not appear within uninfected cells. The virus caused cell lysis of 18 strains of H. circularisquama isolated from various embayments throughout central and western Japan, but did not lyse 24 other phytoplankton species that were tested. To our knowledge, this is the first report of a virus infecting dinoflagellates which has been isolated and maintained in culture, and our results demonstrate that viruses which infect and cause lysis of dinoflagellates are a component of natural marine viral communities. KEY WORDS: Dinoflagellate · Harmful algal bloom · HcV · Heterocapsa circularisquama · Viral infection Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 23: [103][104][105][106][107][108][109][110][111] 2001 host organisms, viruses may indirectly affect the carbon and nutrient flow in natural aquatic ecosystems (Gobler et al. 1997, Thingstad & Lignell 1997.Despite the importance of the role of algal viruses in marine ecosystem, detailed information on the effect of viral infection on marine phytoplankton is still lacking. The main reason for this is that most studies have been based on ultrastructural observations of field-collected phytoplankton cells, while few algal host-virus systems have been successfully cultured and studied in the laboratory to date (Waters & Chan 1982, Cottrell & Suttle 1991, Van Etten et al. 1991, Suttle & Chan 1995, Jacobsen et al. 1996, Gastrich et al. 1998, Nagasaki et al. 1999a. In the present paper, we describe the isolation and general characteristics of a virus infecting and lysing Heterocapsa circularisquama. To our knowledge, this is the first report on a virus infecting a dinoflagellate which has been isolated and maintained in culture. MATERIALS AND METHODSAlgal cultures and growth conditions. The main strains of Heterocapsa circularisquama, HU9433-P and HA92-1, used throughout this study were isolated from Uranouchi Bay (Kochi Prefecture, Japan) in 1994 and Ago Bay (Mie Prefecture, Japan) in 1992, respectively (Table 1). Both strains were extracellularly axenic, but HA92-1 harbors bacteria within the cell probably in a symbiotic relationship. In contrast, HU9433-P was obtained by selecting a non-bacteria-harboring cell from the original strain isolated from Uranouchi Bay (HU9433), most of whose cells harbor some bacteria within them, and w...

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