Feeding by red-tide dinoflagellates on the cyanobacterium Synechococcus

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 x 10(6) cells ml(-1), the ingestion rates of these cultured red-tide dinoflagellates on Synechococcus sp. (1.0 to 64.2 cells dinoflagellate(-1) h(-1)) generally increased with increasing size of the dinoflagellate predators (equivalent spherical diameters = 5.2 to 38.2 mu m). 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 x 10(6) cells ml(-1). The maximum ingestion and clearance rates of P. micans on Synechococcus sp. (38.2 cells dinoflagellate(-1) h(-1) and 4.3 mu l dinoflagellate(-1) h(-1)) were much higher than those of P. donghaiense on the same prey species (7.7 cells dinoflagellate(-1) h(-1) and 2.6 mu l dinoflagellate(-1) h(-1)). 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. (R 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.