CHARACTERIZATION OF HaRNAV, A SINGLE‐STRANDED RNA VIRUS CAUSING LYSIS OF HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE)¹

Abstract

HaRNAV, a novel virus that infects the toxic bloom‐forming alga Heterosigma akashiwo (Hada) Hada ex Hada et Chihara, was characterized based on morphology, pathology, nucleic acid type, structural proteins, and the range of host strains that it infects. HaRNAV is a 25‐nm single‐stranded RNA (ssRNA) virus with a genome size of approximately 9100 nucleotides. This is the first report of an ssRNA virus that causes lysis of a phytoplankton species. The virus particle is sensitive to chloroform and contains at least five structural proteins ranging in apparent size from 24 to 34 kDa. HaRNAV infection causes swelling of the endoplasmic reticulum and progeny virus particles assemble in the cytoplasm of the host, frequently in crystalline arrays. The infectivity of HaRNAV was tested against 15 strains of H. akashiwo isolated from Japanese waters, the Northeast Pacific, and the Northwest Atlantic. HaRNAV caused lysis of three strains from the Northeast Pacific and two strains from Japan but none from the Northwest Atlantic. The characterization of HaRNAV demonstrates that HaRNAV is a novel type of phytoplankton virus but has some similarities with plant viruses belonging to the Sequiviridae and to other known ssRNA viruses. Further genomic analysis, however, is necessary to determine any phylogenetic relationships. The discovery of HaRNAV emphasizes the diversity of H. akashiwo viral pathogens and, more importantly, algal–virus pathogens and the complexity of virus–host interactions in the environment.

Isolation and Characterization of a Novel Single-Stranded RNA Virus Infectious to a Marine Fungoid Protist, Schizochytrium sp. (Thraustochytriaceae, Labyrinthulea)

“…2C). Vacuolation of the cytoplasm and the appearance of fibrils within vacuoles were also observed with HaRNAV (61).…”

“…2E) in the host cell's cytoplasmic area. Both crystalline array formation and unordered aggregation within the cytoplasm are common features of several ssRNA viruses infectious to plants (36,56), picornaviruses infectious to animals (11,24), and marine algal ssRNA viruses (HaRNAV and HcRNAV) (61,63). It was also notable that the virus particles were concentrated along the intracellular membrane structures (Fig.…”

“…5); thus, the latent period of SssRNAV was estimated to be Ͻ8 h. The burst sizes estimated from the experiments ranged from 5.8 ϫ 10 3 to 6.4 ϫ 10 4 infectious units cell Ϫ1 . When SssRNAV was compared with the other ssRNA viruses infecting microalgae, the latent period of SssRNAV was found to be much shorter than those of HaRNAV (Ͻ12 days) (61) and HcRNAV (1 to 2 days) (63), and the burst size was found to be similar to that of HcRNAV (3.4 ϫ 10 3 to 16 ϫ 10 3 infectious units cell Ϫ1 ) (63). TEM observations revealed that Ͼ3.4 ϫ 10 3 SssRNAV particles were scattered in a thin section of an infected cell 8 h after virus inoculation (Fig.…”

“…So far, three RNA viruses infecting marine eukaryotic microalgae have been isolated; two of them are single-stranded RNA (ssRNA) viruses (HaRNAV and HcRNAV), and one is a double-stranded RNA (dsRNA) virus (MpRNAV). HaRNAV is infectious to one of the most noxious bloomforming phytoflagellates, Heterosigma akashiwo (Raphidophyceae), and has an ssRNA genome that is 9.1 kb long (61). HcRNAV is infectious to the bivalve-killing dinoflagellate Heterocapsa circularisquama and has an approximately 4.4-kb ss-RNA genome (63).…”

Effects of solar ultraviolet radiation on photosynthesis of the marine red tide alga Heterosigma akashiwo (Raphidophyceae)

“…At the same time, it had been shown allelopathic effects towards other algae, specifically diatoms [30]. The growth and toxicity of H. akashiwo can be mediated by hydrogen peroxide [24,31] or viruses [32,33]. In addition, and more relevant to the blooms H. akashiwo in a climate change environment, [34] showed that growth and toxicity of this alga was markedly influenced by changes in temperature and light intensity.…”