No abstract

Recent observations that viruses are very abundant and biologically active components in marine ecosystems suggest that they probably influence various biogeochemical and ecological processes. In this study, the population dynamics of the harmful bloom-forming phytoplankton Heterosigma akashiwo (Raphidophyceae) and the infectious H. akashiwo viruses (HaV) were monitored in Hiroshima Bay, Japan, from May to July 1998. Concurrently, a number of H. akashiwo and HaV clones were isolated, and their virus susceptibilities and host ranges were determined through laboratory cross-reactivity tests. A sudden decrease in cell density of H. akashiwo was accompanied by a drastic increase in the abundance of HaV, suggesting that viruses contributed greatly to the disintegration of the H. akashiwo bloom as mortality agents. Despite the large quantity of infectious HaV, however, a significant proportion of H. akashiwo cells survived after the bloom disintegration. The viral susceptibility of H. akashiwo isolates demonstrated that the majority of these surviving cells were resistant to most of the HaV clones, whereas resistant cells were a minor component during the bloom period. Moreover, these resistant cells were displaced by susceptible cells, presumably due to viral infection. These results demonstrated that the properties of dominant cells within the H. akashiwo population change during the period when a bloom is terminated by viral infection, suggesting that viruses also play an important role in determining the clonal composition and maintaining the clonal diversity of H. akashiwo populations. Therefore, our data indicate that viral infection influences the total abundance and the clonal composition of one host algal species, suggesting that viruses are an important component in quantitatively and qualitatively controlling phytoplankton populations in natural marine environments.Viruses are now recognized as the most abundant and biologically active components of marine ecosystems (1, 24). Field studies indicate that the majority are probably bacterial viruses, i.e., bacteriophages (5, 33), but viruses and viruslike particles have been observed in many phytoplankton species and in a wide range of natural seawater samples (8,28,32). These observations have led to increased interest in the impact of viral infection on the population dynamics and community structure of marine phytoplankton. Several studies have suggested that viruses can be significant agents of phytoplankton mortality. For example, an addition of native virus concentrates reduced phytoplankton biomass and primary productivity under experimental laboratory conditions (28, 29). Also, electron microscopic observations have shown that the proportion of cells harboring viruslike particles or the abundance of viruslike particles within the water column increases in the final stages of blooms (2,3,16,17). In contrast to these postulates, a few reports have demonstrated that viruses are probably not responsible for a large proportion of host mortality (33,36). Wat...

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...

Several viruses specifically infecting the harmful bloom-forming raphidophyte Heterosigma akashiwo (Hada) Hada have been found recently. It has been reported that infection of a double-stranded DNA (dsDNA) virus (HaV) affects both the biomass and clonal composition of H. akashiwo blooms. To clarify the relationship between H. akashiwo and its viruses, both algal and viral dynamics were monitored in Hiroshima Bay, Japan from May through July 2000. To minimize any underestimation or overlooking of viruses lytic to H. akashiwo, 4 host clones with different virus sensitivity spectra were used for their enumeration and isolation. Because all 65 viral clones obtained were stainable with DAPI, the most dominant viruses lytic to H. akashiwo assessed during the survey were considered to be dsDNA viruses. The abundance of viruses lytic to H. akashiwo monitored by means of the most probable number (MPN) method using each host clone showed its own dynamics pattern, but the viruses shared similar trends with each other, exhibiting a marked increase accompanied by a sudden decrease in host abundance. Thus, different types of viruses lytic to H. akashiwo are considered to have coexisted and simultaneously affected the bloom to cause its decline. Based on the results of laboratory cross-reactivity tests between 90 H. akashiwo clones and 65 virus clones isolated from the bloom, they were divided into 6 and 3 groups, respectively, showing their high diversity with regard to their virus sensitivity and host specificity. Based on the viral dynamics and changes in host abundance and clonal composition from the peak over the end of the bloom, we concluded that the viral infection was one of the most important factors determining quantity (biomass) and quality (clonal composition) of the H. akashiwo population. KEY WORDS: Viral infection · Heterosigma akashiwo · Harmful algal bloom · Algal virus · Host clonal compositionResale or republication not permitted without written consent of the publisher

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.