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Applied and Environmental Microbiology, July 2005, p. 3528-3535, Vol. 71, No. 7
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.7.3528-3535.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Previously Unknown Virus Infects Marine Diatom

Keizo Nagasaki,^1* Yuji Tomaru,¹ Yoshitake Takao,² Kensho Nishida,^1, Yoko Shirai,¹ Hidekazu Suzuki,³ and Tamotsu Nagumo⁴

Harmful Algal Bloom Division, National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Hiroshima, Japan,¹ Department of Biology, Faculty of Science and Engineering, Konan University, Kobe, Japan,² Aoyama Gakuin Senior High School, Tokyo, Japan,³ Department of Biology, Nippon Dental University, Tokyo, Japan⁴

Received 16 November 2004/ Accepted 4 January 2005

Diatoms are a major phytoplankton group that play important roles in maintaining oxygen levels in the atmosphere and sustaining the primary nutritional production of the aquatic environment. Among diatoms, the genus Chaetoceros is one of the most abundant and widespread. Temperature, climate, salinity, nutrients, and predators were regarded as important factors controlling the abundance and population dynamics of diatoms. Here we show that a viral infection can occur in the genus Chaetoceros and should therefore be considered as a potential mortality source. Chaetoceros salsugineum nuclear inclusion virus (CsNIV) is a 38-nm icosahedral virus that replicates within the nucleus of C. salsugineum. The latent period was estimated to be between 12 and 24 h, with a burst size of 325 infectious units per host cell. CsNIV has a genome structure unlike that of other viruses that have been described. It consists of a single molecule of covalently closed circular single-stranded DNA (ssDNA; 6,005 nucleotides), as well as a segment of linear ssDNA (997 nucleotides). The linear segment is complementary to a portion of the closed circle creating a partially double-stranded genome. Sequence analysis reveals a low but significant similarity to the replicase of circoviruses that have a covalently closed circular ssDNA genome. This new host-virus system will be useful for investigating the ecological relationships between bloom-forming diatoms and other viruses in the marine system. Our study supports the view that, given the diversity and abundance of plankton, the ocean is a treasury of undiscovered viruses.

Present address: Hiroshima Prefectural Institute of Industrial Science and Technology, 3-10-32 Kagamiyama, Higashi-Hiroshima 739-0046, Japan.

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