Morphological characterization of Symbiodinium minutum and S. psygmophilum belonging to clade B

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  • ABSTRACT

    Dinoflagellates in the genus Symbiodinium are known as zooxanthellae and exist as either symbiotic or free-living forms. Among this genus, two species, Symbiodinium minutum and Symbiodinium psygmophilum, were recently established based on genetic characters. However, some critical morphological characters of these species, such as plate formulae and related diagnoses have not been provided yet. In this study, we analyzed the morphology of S. minutum and S. psygmophilum by scanning and transmission electron microscopy. The S. minutum had the Kofoidian plate formula consisting of a small plate (x), elongated amphiesmal vesicle (EAV), 4ʹ, 5a, 8ʹʹ, 7s, two cingulum rows, 18-20c, 6ʹʹʹ, and 2ʹʹʹʹ, while S. psygmophilum had x, EAV, 4ʹ, 5a, 8ʹʹ, 7-10s, two cingulum rows, 20-22c, 5-6ʹʹʹ, and 1ʹʹʹʹ. These plate formulae are different from any other reported Symbiodinium species. In addition, both species had a pentagonal 1a plate and a hexagonal 2a plate, while other known Symbiodinium species had a hexagonal 1a plate and a pentagonal 2a plate. Thus, we confirm the species status of S. minutum and S. psygmophilum based on morphological and genetic characters and report the detailed morphological characteristics of these two species.


  • KEYWORD

    dinoflagellate , Dinophyceae , eyespot , protist , Symbiodiniaceae , taxonomy

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  • [Table 1.] Species, strain name, clade, location of collection (LC), host and collection information of 2 Symbiodinium clade B species obtained from the National Center for Marine Algae and Microbiota (formerly the Provasoli-Guillard National Center for Culture of Marine Phytoplankton)
    Species, strain name, clade, location of collection (LC), host and collection information of 2 Symbiodinium clade B species obtained from the National Center for Marine Algae and Microbiota (formerly the Provasoli-Guillard National Center for Culture of Marine Phytoplankton)
  • [Table 2.] Comparison of morphologically reported Symbiodinium species based on figures obtained under scanning electron microscopy
    Comparison of morphologically reported Symbiodinium species based on figures obtained under scanning electron microscopy
  • [Fig. 1.] Scanning electron micrographs of Symbiodinium minutum motile cells. (A) Ventral view showing the episome, cingulum (c), sulcus (s), peduncle (PE), and hyposome. (B) Ventral-left lateral view showing the episome, cingulum (c), and hyposome. (C) Dorsal view showing the episome, cingulum (c), and hyposome. (D) Ventral-right lateral view showing the episome, cingulum (c), and hyposome. (E) Apical view showing the episome and elongated amphiesmal vesicle (EAV). (F) Apical view showing the EAV with small knobs. (G) Antapical view showing the hyposome. (H) Antapical-ventral view showing the sulcus (s) and peduncle (PE). S.p., posterior sulcus. Scale bars represent: A-H, 1 μm.
    Scanning electron micrographs of Symbiodinium minutum motile cells. (A) Ventral view showing the episome, cingulum (c), sulcus (s), peduncle (PE), and hyposome. (B) Ventral-left lateral view showing the episome, cingulum (c), and hyposome. (C) Dorsal view showing the episome, cingulum (c), and hyposome. (D) Ventral-right lateral view showing the episome, cingulum (c), and hyposome. (E) Apical view showing the episome and elongated amphiesmal vesicle (EAV). (F) Apical view showing the EAV with small knobs. (G) Antapical view showing the hyposome. (H) Antapical-ventral view showing the sulcus (s) and peduncle (PE). S.p., posterior sulcus. Scale bars represent: A-H, 1 μm.
  • [Fig. 2.] Drawings of Symbiodinium minutum motile cells showing the external morphology. (A) Ventral view. (B) Dorsal view. (C) Apical view. (D) Antapical view. c, cingulum; s, sulcus; S.p., posterior sulcus. Scale bars represent: A-D, 1 μm.
    Drawings of Symbiodinium minutum motile cells showing the external morphology. (A) Ventral view. (B) Dorsal view. (C) Apical view. (D) Antapical view. c, cingulum; s, sulcus; S.p., posterior sulcus. Scale bars represent: A-D, 1 μm.
  • [Fig. 3.] Transmission electron micrographs of Symbiodinium minutum cells. (A) Transverse section of a mastigote cell showing the position of the nucleus possessing chromosomes (black arrowheads) in the middle of cell, chloroplasts (c), two stalked pyrenoid (PY), type E eyespot (stigma, white arrow) near the cell’s surface, lipid globules (L), and starch (st). (B) Type E eyespot (white arrow) consisting of multiple layers of rectangular electrontranslucent vesicles, or crystalline deposits. (C) Serial sectioning showing the large number of lipid globules (L). (D) Pyrenoid with two stalks (white arrowheads) connected to chloroplasts around the cell’s surface and surrounded by a distinct polysaccharide cap. Scale bars represent: A & C, 1 μm; B & D, 0.5 μm.
    Transmission electron micrographs of Symbiodinium minutum cells. (A) Transverse section of a mastigote cell showing the position of the nucleus possessing chromosomes (black arrowheads) in the middle of cell, chloroplasts (c), two stalked pyrenoid (PY), type E eyespot (stigma, white arrow) near the cell’s surface, lipid globules (L), and starch (st). (B) Type E eyespot (white arrow) consisting of multiple layers of rectangular electrontranslucent vesicles, or crystalline deposits. (C) Serial sectioning showing the large number of lipid globules (L). (D) Pyrenoid with two stalks (white arrowheads) connected to chloroplasts around the cell’s surface and surrounded by a distinct polysaccharide cap. Scale bars represent: A & C, 1 μm; B & D, 0.5 μm.
  • [Fig. 4.] Scanning electron micrographs of Symbiodinium psygmophilum motile cells. (A) Ventral view showing the episome, cingulum (c), sulcus (s), peduncle (PE), and hyposome. (B) Ventral-left lateral view showing the episome, cingulum (c), and hyposome. (C) Dorsal view showing the episome, cingulum (c), and hyposome. (D) Ventral-right lateral view showing the episome, cingulum (c), and hyposome. (E) Apical view showing the episome and elongated amphiesmal vesicle (EAV). (F) Apical view showing the EAV with small knobs. (G) Antapical view showing the hyposome. (H) Antapical-ventral view showing the sulcus (s) and peduncle (PE). S.p., posterior sulcus. Scale bars represent: A-H, 1 μm.
    Scanning electron micrographs of Symbiodinium psygmophilum motile cells. (A) Ventral view showing the episome, cingulum (c), sulcus (s), peduncle (PE), and hyposome. (B) Ventral-left lateral view showing the episome, cingulum (c), and hyposome. (C) Dorsal view showing the episome, cingulum (c), and hyposome. (D) Ventral-right lateral view showing the episome, cingulum (c), and hyposome. (E) Apical view showing the episome and elongated amphiesmal vesicle (EAV). (F) Apical view showing the EAV with small knobs. (G) Antapical view showing the hyposome. (H) Antapical-ventral view showing the sulcus (s) and peduncle (PE). S.p., posterior sulcus. Scale bars represent: A-H, 1 μm.
  • [Fig. 5.] Drawings of Symbiodinium psygmophilum motile cells showing the external morphology. (A) Ventral view. (B) Dorsal view. (C) Apical view. (D) Antapical view. c, cingulum; s, sulcus; S.p., posterior sulcus. Scale bars represent: A-D, 1 μm.
    Drawings of Symbiodinium psygmophilum motile cells showing the external morphology. (A) Ventral view. (B) Dorsal view. (C) Apical view. (D) Antapical view. c, cingulum; s, sulcus; S.p., posterior sulcus. Scale bars represent: A-D, 1 μm.
  • [Fig. 6.] Transmission electron micrographs of Symbiodinium psygmophilum cells. (A) Transverse section of a mastigote cell showing the pyrenoid (PY), nucleus, chloroplasts (c), type E eyespot (stigma), lipid globules (L), and starch (st). (B) Type E eyespot consisting of multiple layers of rectangular electron-translucent vesicles, or crystalline deposits (stigma, white arrow). (C) Cell with many chromosomes in the nucleus. (D) Single pyrenoid with two stalks (white arrowheads), located in the central part of each cell and surrounded by a distinct polysaccharide cap. Scale bars represent: A & C, 1 μm; B & D, 0.5 μm.
    Transmission electron micrographs of Symbiodinium psygmophilum cells. (A) Transverse section of a mastigote cell showing the pyrenoid (PY), nucleus, chloroplasts (c), type E eyespot (stigma), lipid globules (L), and starch (st). (B) Type E eyespot consisting of multiple layers of rectangular electron-translucent vesicles, or crystalline deposits (stigma, white arrow). (C) Cell with many chromosomes in the nucleus. (D) Single pyrenoid with two stalks (white arrowheads), located in the central part of each cell and surrounded by a distinct polysaccharide cap. Scale bars represent: A & C, 1 μm; B & D, 0.5 μm.