Co-occurrence of DNA- and RNA-viruses infecting the bloom-forming dinoflagellate, Heterocapsa circularisquama, on the Japan coast

In western Japan, Heterocapsa circularisquama blooms occur almost every year. Two distinct viruses infectious to H. circularisquama, a large double-stranded DNA (dsDNA) virus, 'HcV', and a small single-stranded RNA (ssRNA) virus, 'HcRNAV', were found and characterized. Here we report the co-occurrence of the two viruses in Hiroshima Bay, Japan, during 2006. Viruses isolated using H. circularisquama strains HU9433-P and MZ2 (the hosts) were large dsDNA viruses (ca. 0.2 micro m in diameter) that accumulated in the host cytoplasm. Pulsed-field gel electrophoresis analysis showed their genome sizes were ca. 388 kbp, comparable to the previously reported HcV strain. Small ssRNA viruses were also isolated (using the host strain HCLG-1) at ca. 30 nm in diameter and formed crystalline arrays and/or were randomly accumulated in the host cytoplasm, and their partial genome sequences were highly similar to a previously reported HcRNAV strain. This is the first report of the co-occurrence of HcV and HcRNAV in natural water and their successful isolation. The data suggest the two distinct viruses may cooperatively affect the dynamics of H. circularisquama blooms.