Ultrastructural and molecular characterization of cyanobacterial symbionts in Dictyocoryne profunda (polycystine radiolaria)
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Cyanobacterial symbionts were detected in the extracytoplasm of the polycystine radiolarian Dictyocoryne profunda Ehrenberg. The bacterial symbionts were observed as numerous spherical bodies ~0.5–1.0 μm in diameter under transmission electron microscopy. They were present in a very restricted location close to the periphery of the host radiolarian shell, adjacent to the central capsular wall. Several cells of them may have been in the process of cell division or just divided. The symbionts had thylakoid-like structures, which ran around the cell periphery in two or three concentric layers. Based on the small subunit ribosomal DNA (16S rDNA) phylogenetic analyses, the intracellular symbiotic bacteria grouped with cyanobacteria belonging to the genus Synechococcus. Three sequences, one from each of three specimens of D. profunda, collected in March/October 2009 and March 2010 from the East China Sea, were the same and branched within Synechococcus clade II, that is characterized by strains with low amounts of phycourobilin (PUB).
KeywordsCyanobacteria Dictyocoryne profunda Radiolaria Symbiont Synechococcus.
We would like to acknowledge A. Takemura, Y. Nakano, and S. Nakamura of the University of the Ryukyus for their support and helpful discussions. We also wish to thank D. Honda of the Konan University for his valuable advice and suggestions. This study was financially supported by the grant-in-aid from JSPS Grants 19540491 and 23540545 to O. Takahashi.
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