Quantitative and qualitative impacts of viral infection on a Heterosigma akashiwo (Raphidophyceae) bloom in Hiroshima Bay, Japan

Abstract

Several viruses specifically infecting the harmful bloom-forming raphidophyte Hetero- sigma 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 accom- panied 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 con- cluded that the viral infection was one of the most important factors determining quantity (biomass) and quality (clonal composition) of the H. akashiwo population.