Isolation of a virus infecting the novel shellfish-killing dinoflagellate Heterocapsa circularisquama

Abstract

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...

Isolation and Characterization of a Novel Single-Stranded RNA Virus Infectious to a Marine Fungoid Protist, Schizochytrium sp. (Thraustochytriaceae, Labyrinthulea)

“…So far, more than 13 viruses infecting marine microalgae have been isolated and characterized (5). The majority of these viruses are large (100-to 200-nm) double-stranded DNA viruses, and they are either included in the family Phycodnaviridae (5,58,65) or considered most likely to belong to this family based on some characteristics (7,16,23,43,57,62).…”

Isolation and Physiological Characterization of a New Algicidal Virus Infecting the Harmful Dinoflagellate Heterocapsa pygmaea

“…As far as the authors know, this is one of only a few reports describing features of a virus infecting dinoflagellates (Alveolata) to be made after the isolations of HcV and HcRNAV, which infect Heterocapsa circularisquama (Nagasaki et al, 2003;Tarutani et al, 2001;Tomaru et al, 2004a). The virion size, shape, the cytoplasm as accumulating site and large dsDNA genome suggest that HpygDNAV is new member of the NCLDV group (Iyer et al, 2006).…”

“…Among these species, the first and second cultured dinoflagellate-infecting viruses reported were characterized as a large icosahedral double-stranded (ds) DNA virus, Heterocapsa circularisquama virus (HcV, Tarutani et al, 2001;Nagasaki et al, 2003), and a small icosahedral singlestranded (ss) RNA virus, Heterocapsa circularisquama RNA virus (HcRNAV, Tomaru et al, 2004), infecting Heterocapsa circularisquama. Prior to the isolation of these viruses, viral infection in dinoflagellate has been scarcely investigated; there were only three reports concerning the VLPs observed in dinoflagellate by transmission electron microscope in the 1970's (Franca, 1976;Sicko-Goad and Walker, 1979;Soyer, 1978).…”

“…1A, B). Further cloning of a virus strain was performed according to the method of Tarutani et al (2001), and the isolated virus was named as HpygDNAV. The host range of HpygDNAV was examined by adding 50 µl of the lysate to each 1 ml culture of exponentially growing algal strains listed in Table 1.…”

“…Cultures containing 500 ml of exponentially growing host cells were inoculated with 20 ml of lysate containing HpygDNAV at a viral titer of 7.0 × 10 4 estimated by most probable number (MPN); cell density and virus titer were then respectively measured by light microscopy and the extinction dilution method (Tarutani et al, 2001) every 24 h until 140 hours postinoculation (hpi; Fig. 2).…”