Morphological variation, ultrastructure, pigment composition and phylogeny of the star-shaped dinoflagellate Asterodinium gracile (Kareniaceae, Dinophyceae)

Morphological variation, ultrastructure, pigment composition and phylogeny of an oceanic, star-shaped, unarmored dinoflagellate Asterodinium gracile were examined using a culture established from Nagasaki, Japan, in 2017. Our examinations were based on LM, SEM, TEM, high-performance liquid chromatography (HPLC) and molecular phylogeny inferred from nuclear-encoded internal transcribed spacer (ITS) and 28S rDNA and chloroplast-encoded 16S rDNA sequences. Typical cells were stellate, possessing an apical, two lateral, and two antapical extensions. Cells ranged in size from 62.5 to 105.6 pm long and 21.2 to 39.5 mu m in singular width. The length and orientation of cellular extensions were variable; that is, two lateral extensions were usually symmetrically bent or retracted. Ellipsoidal cells without extensions were also observed. The apical structure complex was straight or slightly sigmoid. A spherical nucleus was located at the left side in the hyposome. The numerous discoid, yellowish-brown chloroplasts were variable in size and distributed also in the cell extensions. Among Kareniaceae, the lack of a pyrenoid in Asterodinium is unique. Nuclear-encoded ITS and 28S phylogenies suggested that A. gracile is closely related to Karenia papilionacea in the Kareniaceae, but its affinity to Brachidinium capitatum, another species with extensions, was unclear. A phylogeny of 16S rDNA showed that chloroplasts of A. gracile are related to other kareniaceans derived from a haptophyte. These chloroplasts were of the fucoxanthin type, composed of pigments similar to those in other Kareniaceae; however, the detection of 19'-hexanoyloxyfucoxanthin as a major carotenoid, the low amount of fucoxanthin, and lack of 19'-butanoyloxyfucoxanthin in A. gracile were unusual for the family.