Intracellular ultrastructural characterization of Azadinium and Amphidoma (Amphidomataceae, Dinophyceae), including toxigenic and non-toxigenic strains.

The marine toxigenic dinoflagellate family Amphidomataceae, comprising Amphidoma and Azadinium, has been primarily characterized by thecal morphology examined by SEM. To further clarify the detailed morphology, the intracellular ultrastructure of five Azadinium species, encompassing two toxigenic species (Azadinium poporum and Az. spinosum) and three non-toxigenic species (Az. inconspicuum, Az. dalianense, and Az. zhuanum), was examined using TEM. The ultrastructure of two Amphidoma species was also examined for comparison. All species exhibited typical dinoflagellate organelles, i.e., nucleus, fibrous vesicles, and trichocysts. Vesicles containing crystalline structures were found in all strains. The type of pyrenoid varied among Azadinium species; a stalked pyrenoid from the chloroplast in Az. spinosum, bulged in Az. dalianense and Az. inconspicuum, stalked or bulged in Az. poporum, and embedded in Az. zhuanum. Among the bulged pyrenoids, that of Az. inconspicuum was surrounded by thylakoids, whereas that of Az. dalianense lacked underlying thylakoids. Such intrageneric variations of pyrenoids may provide a useful diagnostic character for species discrimination. Cytoplasmic invaginations that reported in Amphidoma species were not observed in pyrenoids of five Azadinium species. Lipofuscin-like granules exhibiting broad-wavelength autofluorescence observed under LM, likely lipidic bodies associated with multilayered membranes observed under TEM, were found in all species. A microtubular strand and electron-opaque vesicles associated with the peduncle, as well as mucocysts surrounding a canal, were observed for the first time in the Amphidomataceae. The absence of a striated root connective in the flagellar root, as reported in Amphidoma, is consistent with a phylogenetic affinity between the Amphidomataceae and Peridiniales.