Feeding by heterotrophic protists and copepods on the photosynthetic dinoflagellate Azadinium cf. poporum from western Korean waters

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.