Dynamics of Heterocapsa circularisquama (Dinophyceae) and its viruses in Ago Bay, Japan

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 virus- like particles (VLPs). Based on transmission electron microscopic (TEM) observation, morphological resemblance between these VLPs and the single-stranded RNA (ssRNA) virus infecting H. circularis- quama (HcRNAV: H. circularisquama RNA virus) isolated from the bloom was noticeable. The fluc- tuation patterns of the viruses indicated that at least 2 distinct types of virus with different host speci- ficity 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.