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English: Phylogenetic identity of Pseudoblepharisma tenue (Heterotrichea > Blepharismidae) symbiotic partners, cellular localization of the endosymbionts, and details of “Ca. Thiodictyon intracellulare” (Chromatiales > Chromatiaceae). ('A) Maximum likelihood phylogenetic tree of the Heterotrichea based on the concatenated nuclear rRNA gene operon (including sequences of 18S rRNA, ITS1, 5.8S rRNA, ITS2, and D1D2-28S rRNA regions) and the mitochondrial COI gene. P. tenue is sister to Spirostomum species with moderate support. (B) Maximum likelihood phylogenetic tree of the Chlorellaceae based on the chloroplast-encoded rbcL gene. The green algal endosymbionts of P. tenue are identical in their rbcL gene sequence to Chlorella sp. K10, an endosymbiont of Hydra viridissima. (C) Maximum likelihood tree of the Chromatiaceae based on the 16S rRNA gene. The purple bacterial endosymbionts are most closely related to Thiodictyon species. (D) Fluorescence signal of probe Thio643-Cy3 in spread-out bacterial cells from P. tenue cytoplasm. Probe-conferred fluorescence (shown in pseudocolor) decreases with increasing formamide (FA) concentration (expected binding behavior). (E) Confocal fluorescent (CLSM) and transmitted light [DIC (differential interference contrast)] micrographs of a P. tenue cell displaying the FISH signal for probe Thio643-Cy3 (cyan; pseudocolor) and the autofluorescence of green algal chlorophyll (red; pseudocolor). (F) Transmission electron micrograph of “Ca. Thiodictyon intracellulare” and starch granules in the cytoplasm of P. tenue. The bacterial cytoplasm is packed with vesicular chromatophores and surrounded by three membranes. (G) Absorbance spectra of purple and green endosymbionts recorded with hyperspectral microscopy. Note the peaks in the infrared range (>700 nm) typical for bacteriochlorophyll. Negative absorbance values in the region >900 nm are artifacts probably based on a low signal-to-noise ratio. Representative sample areas for purple cells (PC) and green cells (GC) are shown in the inset. Scale bars, 10 μm (E) and 100 nm (F).