No abstract

We studied the ecological relationships between the bloom-forming dinoflagellate Heterocapsa circularisquama and its infectious viruses in field surveys conducted in western Japan. The occurrence of H. circularisquama blooms in Imari Bay during 2002 and in Ago Bay during 2002 and 2004 was accompanied by specific increase in abundance of viruses lytic to H. circularisquama. Using northern dot-blot analysis, approximately 96% of the clonal virus isolates collected in the field surveys positively reacted with a molecular probe specific for HcRNAV (H. circularisquama RNA virus); hence, viral impacts on H. circularisquama population observed in these field surveys are considered largely due to HcRNAV and/or its closely related viruses. The dynamics of type UA viruses and type CY viruses having complementary host ranges to H. circularisquama clones were different in each survey and considered to reflect fluctuations in abundance of their suitable host cells in situ. The dynamics of H. circularisquama and its viruses in Ago Bay from 2002 to 2004 suggests the concentration of HcRNAV in the sediment prior to the host's blooming season is a significant factor in determining the size and length of the H. circularisquama blooms. These results support the hypothesis that HcRNAV infection is one of the significant factors affecting the population dynamics of H. circularisquama in both quantity (biomass) and quality (clonal composition).

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.

Diatoms are considered the most successful and widespread group of photosynthetic eukaryotes. Their contribution to primary production is remarkably significant to the earth's ecosystems. Diatoms are composed of two orders: Centrales and Pennales. Thus far, viruses infecting centric diatom species have been isolated and characterized; however, viruses infecting pennates have not been reported. Here, we describe the first isolations and preliminary characterizations of two distinct pennate diatom viruses, AglaRNAV (31 nm in diameter, accumulates in the host cytoplasm) and TnitDNAV (35 nm in diameter, accumulates in the host nuclei) infecting Asterionellopsis glacialis and Thalassionema nitzschioides, respectively. Their genomes contain a single-stranded RNA of approximately 9.5 kb, and a closed, circular single-stranded DNA of approximately 5.5 kb harboring a partially double-stranded region, respectively. Further analysis of these viruses may elucidate many aspects of diatom host-virus relationships.

HcRNAV is a single-stranded RNA (ssRNA) virus that specifically infects the bivalve-killing dinoflagellate, Heterocapsa circularisquama. HcRNAV strains are grouped into 2 types (UA and CY), based on intra-species host specificity and the amino acid sequence of the major capsid protein (MCP). In the present study, we report the isolation of novel HcRNAV clones (n=51) lytic to the H. circularisquama strains, HU9433-P, HCLG-1, 05HC05 and 05HC06. HcRNAV34, HcRNAV109, HcRNAV641, and HcRNAV659, which displayed lytic activity against the strains, HU9433-P, HCLG-1, 05HC05, and 05HC06, respectively, were selected as typical virus clones and were intensively examined. The infection intensity of each host-virus combination was analyzed by examining the algicidal activity, detecting the intracellular replication of the viral RNA as well as the appearance of host cells with a morphologically abnormal nucleus post-infection. Interestingly, the strains, 05HC05 and 05HC06, were markedly sensitive to HcRNAV641 and HcRNAV659, respectively. Tertiary structural modeling predicted 4 unique amino acid (aa) substitutions in HcRNAV659-MCP to be exposed to an ambient water environment, which contributed towards determining its infection specificity. Neighbor-joining analysis of MCP aa sequences from HcRNAV clones revealed 3 clades, namely, the CY type and the UA1 and UA2 subtypes. The HcRNAV clones lytic to HCLG-1 (ex. HcRNAV109), HU9433-P and 05HC05 (ex. HcRNAV34), and 05HC06 (ex. HcRNAV659) were categorized into CY type, UA1 and UA2 subtypes, respectively. The present study highlights the complexity of the H. circularisquama-HcRNAV host-virus system, i.e., clonal variation, microbial control, and ecology in a natural algal population.