We investigated the feeding by 18 red-tide dinoflagellate species on the cyanobacterium Synechococcus sp. We also calculated grazing coefficients by combining the field data on abundances of the dinoflagellates Prorocentrum donghaiense and P. micans and co-occurring Synechococcus spp. with laboratory data on ingestion rates obtained in the present study. All 17 cultured red-tide dinoflagellates tested (Akashiwo sanguinea, Alexandrium catenella, A. minutum, A. tamarense, Cochlodinium polykrikoides, Gonyaulax polygramma, G. spinifera, Gymnodinium catenatum, G. impudicum, Heterocapsa rotundata, H. triquetra, Karenia brevis, Lingulodinium polyedrum, Prorocentrum donghaiense, P. minimum, P. micans, and Scrippsiella trochoidea) were able to ingest Synechococcus. Also, Synechococcus cells were observed inside the protoplasms of P. triestinum cells collected from the coastal waters off Shiwha, western Korea, during red tides dominated by the dinoflagellate in July 2005. When prey concentrations were 1.1 to 2.3 × 10 6 cells ml ) generally increased with increasing size of the dinoflagellate predators (equivalent spherical diameters = 5.2 to 38.2 µm). The ingestion rates of P. donghaiense and P. micans on Synechococcus sp. increased with increasing mean prey concentration, with saturation occurring at a mean prey concentration of approximately 1.1 to 1.4 × 10 6 cells ml ). The ingestion rates of red-tide dinoflagellates on Synechococcus sp. were comparable to those of the heterotrophic nanoflagellates and ciliates on Synechococcus spp., so far reported in the literature. The calculated grazing coefficients attributable to small Prorocentrum spp. (P. donghaiense + P. minimum) and P. micans on co-occurring Synechococcus spp. were up to 3.6 and 0.15 h -1 , respectively. The results of the present study suggest that red-tide dinoflagellates potentially have a considerable grazing impact on populations of Synechococcus.KEY WORDS: Cyanophyte · Grazing · Harmful algal bloom · Ingestion · Marine · Protist · Red tide Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 41: [131][132][133][134][135][136][137][138][139][140][141][142][143] 2005 dance and/or the primary production of phytoplankton in both coastal and open oceanic waters (Chang et al. 1996, 2003, Karlson et al. 1996, Phlips & Badylak 1996, Agawin et al. 1998, Yahel et al. 1998, Crosbie & Furnas 2001, Arin et al. 2002, Chiang et al. 2002, Stal et al. 2003, Nielsen et al. 2004, Wawrik & Paul 2004. The abundance of Synechococcus spp. often exceeds 10 5 cells ml -1 and sometimes forms red tides (Lindell & Post 1995, Partensky et al. 1996, 1999, Morel 1997, Andreoli et al. 1999, Diaz & Maske 2000, Uysal 2000, Agawin et al. 2003, Murrell & Lores 2004. In oceanic waters, Synechococcus has been thought to be one of the major contributors to CO 2 and nutrient uptake from ambient waters and, in turn, eventually from the atmosphere (Maranon et al. 2003). Therefore, the growth and mortality of Synechococcus are important...

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