Planktonic mixotrophic and heterotrophic dinoflagellates are ubiquitous protists and often abundant in marine environments. Recently many phototrophic dinoflagellate species have been revealed to be mixotrophic organisms and also it is suggested that most dinoflagellates may be mixotrophic or heterotrophic protists. The mixotrophic and heterotrophic dinoflagellates are able to feed on diverse prey items including bacteria, picoeukaryotes, nanoflagellates, diatoms, other dinoflagellates, heterotrophic protists, and metazoans due to their diverse feeding mechanisms. In turn they are ingested by many kinds of predators. Thus, the roles of the dinoflagellates in marine planktonic food webs are very diverse. The present paper reviewed the kind of prey which mixotrophic and heterotrophic dinoflagellates are able to feed on, feeding mechanisms, growth and ingestion rates of dinoflagellates, grazing impact by dinoflagellate predators on natural prey populations, predators on dinoflagellates, and red tides dominated by dinoflagellates. Based on this information, we suggested a new marine planktonic food web focusing on mixotrophic and heterotrophic dinoflagellates and provided an insight on the roles of dinoflagellates in the food web.

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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Survival of free-living and symbiotic dinoflagellates (Symbiodinium spp.) in coral reefs is critical to the maintenance of a healthy coral community. Most coral reefs exist in oligotrophic waters, and their survival strategy in such nutrient-depleted waters remains largely unknown. In this study, we found that two strains of Symbiodinium spp. cultured from the environment and acquired from the tissues of the coral Alveopora japonica had the ability to feed heterotrophically. Symbiodinium spp. fed on heterotrophic bacteria, cyanobacteria (Synechococcus spp.), and small microalgae in both nutrient-replete and nutrient-depleted conditions. Cultured free-living Symbiodinium spp. displayed no autotrophic growth under nitrogen-depleted conditions, but grew when provided with prey. Our results indicate that Symbiodinium spp.'s mixotrophic activity greatly increases their chance of survival and their population growth under nitrogen-depleted conditions, which tend to prevail in coral habitats. In particular, free-living Symbiodinium cells acquired considerable nitrogen from algal prey, comparable to or greater than the direct uptake of ammonium, nitrate, nitrite, or urea. In addition, freeliving Symbiodinium spp. can be a sink for planktonic cyanobacteria (Synechococcus spp.) and remove substantial portions of Synechococcus populations from coral reef waters. Our discovery of Symbiodinium's feeding alters our conventional views of the survival strategies of photosynthetic Symbiodinium and corals. mixotrophy | zooxanthella | coral bleaching | food web | Heterosigma

We investigated the feeding of the small heterotrophic dinoflagellates (HTDs) Oxyrrhis marina, Gyrodinium cf. guttula, Gyrodinium sp., Pfiesteria piscicida, and Protoperidinium bipes on marine heterotrophic bacteria. To investigate whether they are able to feed on bacteria, we observed the protoplasm of target heterotrophic dinoflagellate cells under an epifluorescence microscope and transmission electron microscope. In addition, we measured ingestion rates of the dominant heterotrophic dinoflagellate, Gyrodinium spp., on natural populations of marine bacteria (mostly heterotrophic bacteria) in Masan Bay, Korea in 2006-2007. Furthermore, we measured the ingestion rates of O. marina, G. cf. guttula, and P. piscicida on bacteria as a function of bacterial concentration under laboratory conditions. All HTDs tested were able to feed on a single bacterium. Oxyrrhis marina and Gyrodinium spp. intercepted and then ingested a single bacterial cell in feeding currents that were generated by the flagella of the predators. During the field experiments, the ingestion rates and grazing coefficients of Gyrodinium spp. on natural populations of bacteria were 14-61 bacteria/dinoflagellate/h and 0.003-0.972 day(-1), respectively. With increasing prey concentration, the ingestion rates of O. marina, G. cf. guttula, and P. piscicida on bacteria increased rapidly at prey concentrations of ca 0.7-2.2 x 10(6) cells/ml, but increased only slowly or became saturated at higher prey concentrations. The maximum ingestion rate of O. marina on bacteria was much higher than those of G. cf. guttula and P. piscicida. Bacteria alone supported the growth of O. marina. The results of the present study suggest that some HTDs may sometimes have a considerable grazing impact on populations of marine bacteria, and that bacteria may be important prey.