1. Laboratory of Marine Environmental Microbiology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
2. Department of Biology, Faculty of Science, Yamagata University, Kojirakawa 1-4-12, Yamagata 990-8560, Japan
3. Institute for East China Sea Research, Nagasaki University, 1-14 Bunkyo-mach, Nagasaki 852-8521, Japan
4. Plankton Laboratory, Division of Marine Bioresource and Environmental Science, Graduate School of Fisheries Sciences, Hokkaido University, Minato-cho 3-3-1, Hakodate, Hokkaido 041-8611, Japan