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ABSTRACTSix strains of three different Gonyaulaxspecies were established by isolating cells from the Korean coastal area and the East China Sea, and their morphologies and molecular phylogenies based on SSU and LSU rRNA gene sequences were examined. In addition, the growth responses of the Gonyaulaxspecies to changes in temperature and salinity were investigated. Based on morphological features and phylogenetic positions, Gonyaulax whaseongensisand G. polygrammawere identified, and G. geomunensis sp. nov. is proposed in this study. These species displayed the plate formula typical for Gonyaulax, but G. polygrammaand G. geomunensissometimes showed a small intercalary plate (1a) surrounded by plates 2’, *3’ and 3’’. G. geomunensiswas morphologically characterized by an S-type ventral organization, descending with a displacement of one cingulum width and bearing one, two or three prominent antapical spines of similar size. The cell surface, which was distinct from other Gonyaulaxspecies, was thick and heavily reticulated into numerous polygonal areas. The reticulation was deeply excavated. The phylogenetics revealed that G. geomunensisand G. whaseongensisbelong to different clades, and that there are two ribotypes of G. polygramma, which were morphologically indistinguishable. These species had a close phylogenetic relationship to G. hyalina, and all of them were characterized by dextral torsion. G. whaseongensis, G. polygrammaand G. geomunensishad different growth responses to changes of temperature and salinity, respectively, indicating that morphological and phylogenetic identification of Gonyaulaxspecies can be supported by ecological niches.

ABSTRACTVitamins are an important growth factor for the majority of microalgae. Although the effects of light and prey availability on the growth and feeding of mixotrophic dinoflagellates are relatively well understood, the effects of vitamins are currently poorly understood. We investigated the effects of vitamin conditions on feeding by the red tide-forming dinoflagellate Heterocapsa rotundataSIOHR01 on the cyanobacterium Synechococcussp. CC9311. Furthermore, we explored the growth rates of H. rotundatawith and without added prey under different vitamin conditions and found that H. rotundatais auxotrophic for vitamin B12. Heterocapsa rotundataSIOHR01 was able to ingest Synechococcussp. CC9311 with maximum ingestion and clearance rates, in f/2–Si seawater media, of 0.72 cells predator–1h–1and 28.08 nl predator–1h–1, respectively; these were considerably higher than those in f/2–Si seawater media without B vitamins, which showed a maximum of 0.15 cells predator–1h–1and 2.91 nl predator–1h–1, respectively. However, the mixotrophic growth rate of H. rotundataon Synechococcussp. under each vitamin condition was similar to that of its autotrophic growth rates without added prey under the same vitamin condition. Therefore, this study suggests that while the availability of prey did not enhance the growth of H. rotundata, the concentration of vitamin in the medium affected growth, apparently due to the auxotrophic requirement of the dinoflagellate for vitamin B12.

ABSTRACTChlorarachniophytes are cosmopolitan, typically unicellular marine phototrophic protists with green plastids of secondary endosymbiotic origin. However, there are few studies of their eco-physiology. Bigelowiella natansfeeds mixotrophically on diverse strains of Synechococcus. We investigated feeding by B. natans(Chlorarachniophyceae) on diverse strains of the marine cyanobacterium Synechococcusand quantified the ingestion and clearance rates of B. natanson Synechococcusstrains CC9311 and CC9605 as a function of prey concentration, as well as the growth rates of B. natansfeeding on these Synechococcusstrains. The maximum ingestion rates of B. natanson Synechococcusstrains CC9311 and CC9605 were 2.77 and 2.18 cells predator–1h–1, respectively. The maximum clearance rates of B. natanson SynechococcusCC9311 and CC9605 were 197.2 and 331.5 nl predator–1h–1, respectively. The maximum mixotrophic growth rates of B. natanson SynechococcusCC9311 and CC9605 were 0.38 d–1and 0.28 d–1, respectively, while its autotrophic growth rates were 0.174 ± 0.019 d–1. The results of the present study suggest that B. natansis an effective protistan grazer of Synechococcusand that it obtains a significant ecological advantage from a mixotrophic strategy in marine planktonic food webs.