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.
We explored the interactions between the photosynthetic dinoflagellate Azadinium cf. poporum isolated from Korean waters and potential predators, including engulfment feeders, a pallium feeder, peduncle feeders, and filter feeders. We measured the growth and/or ingestion rates of Oxyrrhis marina, Strobilidium sp., and Acartia spp. on A. cf. poporum as a function of prey concentrations. We also calculated grazing coefficients by using field data on abundance of Strobilidium sp.-sized naked ciliates co-occurring with A. cf. poporum and laboratory data on ingestion rates obtained in this study. Most of the tested organisms were able to feed on A. cf. poporum, but only O. marina, Strobilidium sp., and Acartia spp. showed sustained growth and/or ingestion on A. cf. poporum. Thus, some heterotrophic dinoflagellates using engulfment and filter feeders, such as ciliates and copepods, are likely to be optimal predators, while peduncle-feeding heterotrophic dinoflagellates are unlikely to efficiently feed due to the handling of the theca. The predators had low ratios of maximum growth rate to maximum ingestion rate on A. cf. poporum, as well as low gross growth efficiencies. Therefore, A. cf. poporum appears to be a low-quality prey for the predators tested. Grazing coefficients ranged between 0.052 and 0.446 d −1 , suggesting that Strobilidium sp.-sized naked ciliates may sometimes have a high impact on A. cf. poporum populations, leading to the removal of up to 36% of the population in 1 d. However, the low quality of the prey and predator selectivity in a more complex microbial community may reduce this impact.
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