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...