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Few protistan grazers feed on toxic dinoflagellates, and low grazing pressure on toxic dinoflagellates allows these dinoflagellates to form red-tide patches. We explored the feeding ecology of the newly described heterotrophic dinoflagellate Gyrodinium moestrupii when it fed on toxic strains of Alexandrium minutum, Alexandrium tamarense, and Karenia brevis and on nontoxic strains of A. tamarense, Prorocentrum minimum, and Scrippsiella trochoidea. Specific growth rates of G. moestrupii feeding on each of these dinoflagellates either increased continuously or became saturated with increasing mean prey concentration. The maximum specific growth rate of G. moestrupii feeding on toxic A. minutum (1.60/d) was higher than that when feeding on nontoxic S. trochoidea (1.50/d) or P. minimum (1.07/d). In addition, the maximum growth rate of G. moestrupii feeding on the toxic strain of A. tamarense (0.68/d) was similar to that when feeding on the nontoxic strain of A. tamarense (0.71/d). Furthermore, the maximum ingestion rate of G. moestrupii on A. minutum (2.6 ng C/grazer/d) was comparable to that of S. trochoidea (3.0 ng C/grazer/d). Additionally, the maximum ingestion rate of G. moestrupii on the toxic strain of A. tamarense (2.1 ng C/grazer/d) was higher than that when feeding on the nontoxic strain of A. tamarense (1.3 ng C/grazer/d). Thus, feeding by G. moestrupii is not suppressed by toxic dinoflagellate prey, suggesting that it is an effective protistan grazer of toxic dinoflagellates. [ABSTRACT FROM AUTHOR]

The dinoflagellate Alexandrium spp. have received much attention due to their harmful effects on diverse marine organisms, including commercially important species. For minimizing loss due to red tides or blooms of Alexandrium spp., it is very important to understand the eco-physiology of each Alexandrium species and to predict its population dynamics. Its trophic mode (i.e., exclusively autotrophic or mixotrophic) is one of the most critical parameters in establishing prediction models. However, among the 35 Alexandrium species so far described, only six Alexandrium species have been revealed to be mixotrophic. Thus, mixotrophic ability of the other Alexandrium species should be explored. In the present study, whether each of three Alexandrium species ( A. andersonii , A. affine , and A. fraterculus ) isolated from Korean waters has or lacks mixotrophic ability, was investigated. When diets of diverse algal prey, cyanobacteria, and bacteria sized micro-beads were provided, A. andersonii was able to feed on the prasinophyte Pyramimonas sp., the cryptophyte Teleaulax sp., and the dinoflagellate Heterocapsa rotundata , whereas neither A. affine nor A. fraterculus fed on any prey item. Moreover, mixotrophy elevated the growth rate of A. andersonii . The maximum mixotrophic growth rates of A. andersonii on Pyramimonas sp. under a 14:10 h light/dark cycle of 20 μE m −2 s −1 was 0.432 d −1 , while the autotrophic growth rate was 0.243 d −1 . With increasing mean prey concentration, the ingestion rate of A. andersonii increased rapidly at prey concentrations <650 ng C ml −1 (ca. 16,240 cells ml −1 ), but became saturated at the higher prey concentrations. The maximum ingestion rate by A. andersonii of Pyramimonas sp. was 1.03 ng C predator −1 d −1 (25.6 cells predator −1 d −1 ). This evidence suggests that the mixotrophic ability of A. andersonii should be taken into consideration in predicting the outbreak, persistence, and decline of its harmful algal blooms. [ABSTRACT FROM AUTHOR]

Abstract: The euglenophyte Eutreptiella gymnastica is a common red tide causative species. However, there have been no studies on the grazing impact of heterotrophic protists on this species. To investigate the grazing impact of heterotrophic protists on E. gymnastica, we measured daily the abundances of E. gymnastica and co-occurring potential heterotrophic protistan grazers in Masan Bay, Korea, in August 2004 when an E. gymnastica red tide occurred. In addition, we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes, and Stoeckeria algicida and the naked ciliates Strobilidium sp. (30–40μm in cell length) and Strombidinopsis sp. (70–100μm in cell length) were able to feed on E. gymnastica. We also measured their growth and ingestion rates on E. gymnastica as a function of prey concentration. Finally, we calculated the grazing coefficients by combining field data on the abundance of the heterotrophic dinoflagellate and ciliate grazers and co-occurring E. gymnastica with laboratory data on ingestion rates obtained in this study. The maximum abundance of E. gymnastica in Masan Bay in August, 2004 was 7575cellsml−1, while those of Gyrodinium spp., P. kofoidii, P. bipes, the naked ciliates (≤50μm in cell length), and naked ciliates (>50μm in cell length) were 50, 9, 58, 32, and 3cellsml−1, respectively. The maximum growth rate of G. dominans on E. gymnastica (1.13d−1) was higher than that of O. marina (0.81d−1) or P. bipes (0.77d−1). However, E. gymnastica did not support positive growth of P. kofoidii, Strobilidium sp., and Strombidinopsis sp. (−0.04∼−2.8d−1). The maximum ingestion rates of G. dominans, P. kofoidii, P. bipes, O. marina, and Strobilidium sp. on E. gymnastica (2.1–2.7ngC predator−1 d−1) were similar, but they were much lower than that of Strombidinopsis sp. (156ngCpredator−1 d−1). The calculated grazing coefficients for P. bipes, small heterotrophic Gyrodinium spp. (25–35μm in cell length), naked ciliates (≤50μm in cell length), P. kofoidii, and naked ciliates (>50μm in cell length) on E. gymnastica were up to 0.77, 0.61, 0.22, 0.07 and 0.03d−1, respectively (i.e., up to 54%, 46%, 20%, 7%, and 3% of E. gymnastica populations were removed by the population of each of these heterotrophic protistan grazers in 1d, respectively). The results of the present study suggest that P. bipes, small heterotrophic Gyrodinium spp., and naked ciliates (≤50μm in cell length) sometimes have considerable potential grazing impact on the populations of E. gymnastica. [Copyright &y& Elsevier]