Chloroplast virus causes green-spot disease in cultivated Pyropia of Korea

Viruses are potential regulators of primary production in marine algal populations, which also manipulate the life history and evolution of their hosts. Recent development of an intensive algal aquaculture has enabled emerging viral diseases to spread faster than ever. The pathogen of green-spot disease which causes serious economic loss to Pyropia sea farms in Korea is confirmed for the first time. Ultrastructural study and infection experiment showed that the pathogen is a novel virus, PyroV1 (Pyropia-infecting virus 1), that causes cellular lysis in the blade of the three most commonly cultivated Pyropia species in Korea; Pyropia yezoensis, Pyropia tenera, and Pyropia dentata. Susceptibility test showed that PyroV1 could not infect the conchocelis filaments of the above species nor 16 strains of other red algae, including Pyropia plicata from New Zealand and an Australian strain of Porphyra lucasii. The shape of PyroV1 was isometric, apparently spherical and up to 100 nm in diameter. Chloroplast of the infected cells showed loosely arranged thylakoids and enlarged pyrenoid. Most viral particles were observed around the pyrenoid of the chloroplast and none of them were found in the cytoplasm. In the infected cells, floridean starch granules disappeared and the cytoplasm was filled with swollen vesicles. The burst size was roughly 400-500 infectious units/cell and the latent period was < 36 h. To the best of our knowledge, this is the first report of an isolation of a virus, which infects and causes disease in marine red algae.