Abstract
Bacterial endosymbionts belonging to the family Rickettsiaceae were recently identified in the unicellular green alga Carteria cerasiformis, providing the first molecular evidence of rickettsial endosymbionts within photosynthetic eukaryotes. However, previous morphological studies using transmission electron microscopy (TEM) with conventional chemical fixation did not demonstrate whether the endosymbionts of C. cerasiformis have the diagnostic characteristics of the family Rickettsiaceae. In this study, we observed the rickettsial endosymbionts “MIDORIKO” within C. cerasiformis cells by TEM with high-pressure freezing and freeze-substitution fixation. The rickettsial endosymbionts resided directly in the C. cerasiformis cytoplasm without engulfing or encompassing membranes or vacuoles. The endosymbionts had a Gram-negative cell envelope composed of outer and inner bilayer membranes. The thicknesses of the outer and inner leaflets of the bacterial cell wall were almost identical. These morphological characteristics are consistent with those of the genus Rickettsia, but the cell wall structure differed from that of the genus Orientia within the family Rickettsiaceae.
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Abbreviations
- TEM:
-
Transmission electron microscopy
- FISH:
-
Fluorescence in situ hybridization
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Acknowledgments
This work was supported by Grants-in-Aid for JSPS Fellows (no. 24-9265 to KK), Scientific Research on Innovative Areas (no. 24112707 to HN), and Scientific Research (A) (no. 24247042 to HN) from MEXT/JSPS KAKENHI.
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Handling Editor: Tsuneyoshi Kuroiwa
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Kawafune, K., Sato, M., Toyooka, K. et al. Ultrastructure of the rickettsial endosymbiont “MIDORIKO” in the green alga Carteria cerasiformis as revealed by high-pressure freezing and freeze-substitution fixation. Protoplasma 250, 949–953 (2013). https://doi.org/10.1007/s00709-012-0469-4
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DOI: https://doi.org/10.1007/s00709-012-0469-4