Supplementary Information includes 8 tables (Excel format) that provide information about the
genetic sequences, metadata for statistical analyses and morphological data, and 4 figures in
JPEG format.
Supplementary tables:
Table S1. Information about the primer sets used in this study for PCR amplification of ribosomal
genes of the host Tiarina sp. and the symbiotic microalga Symbiodinium.
Table S2. 18S rDNA sequences of ciliates (order Prorodontida) used for phylogenetic analyses.
GenBank accession numbers in bold indicate the sequences of Tiarina sp. produced in this study.
Table S3. 28S rDNA sequences of Symbiodinium (clades A-H) used for phylogenetic analyses.
GenBank accession numbers in bold indicate the sequences of Symbiodinium clade A produced in this
study.
Table S4. ITS2 sequences of different Symbiodinium sub-clade types within clade A used for
haplotype network reconstruction. GenBank accession numbers in bold indicate the sequences
produced in this study.
Table S5. V9 rDNA sequences of Tiarina sp. (host) and the corresponding symbiont Symbiodinium
used to interrogate the Tara Oceans metabarcoding dataset.
Table S6. Contextual oceanographic parameters and the relative abundance of V9 reads of
Symbiodinium (representing five distinct V9 sequences), Tiarina sp. (genotypes 1 and 2), and Tiarina
fusus at each Tara Oceans station to perform correlation network analyses.
Table S7. Comparison of different morphological characters of seven ciliates within the Colepidae
family, including the newly isolated ciliate Tiarina sp.
Table S8. Significant P-values of the different correlations between environmental physico-chemical
parameters and the distribution and relative abundance of V9 rDNA reads of ciliates and
Symbiodinium.
Supplementary figures:
Figure S1. Schematic diagram of the ribosomal operon (18S and 28S rRNA genes, and ITS)
showing the position of the different primers used in this study to PCR amplify sequences of the
ciliate Tiarina and the symbiotic microalgae Symbiodinium.
Figure S2. Microscopy images of the photosymbiosis between the ciliate Tiarina sp. (the host) and its
intracellular symbiotic microalgae collected in surface oceanic waters. A: One live cell of Tiarina sp.
collected in the Mediterranean Sea (Naples, Italy) in brightfield microscopy (Image courtesy of Diana
Sarno). The ciliate is swimming with the help of numerous cilia at the surface of the cell, and harbours
microalgal cells in the cytoplasm (golden cells). B-D: 3D reconstructions of symbiotic specimens
imaged with Confocal Laser Scanning Microscopy (CLSM). B: Cilia of the ciliate are highlighted in
green (DiOC6). C: The nuclei of the ciliate (cyan) and the symbiotic microalgae (blue) have been
reconstructed from the Hoechst fluorescence signal, and the chloroplasts of the microalgae are
highlighted by the red auto-fluorescence of the chlorophyll. D: Putative cell division events of
Symbiodinium cells are surrounded and could have occured at the early (a), middle (b) and late (c)
stage of the cycle.
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Figure S3. Phylogenetic tree (BioNJ) inferred from partial 28S rDNA sequences of Symbiodinium
clade A (477 aligned nucleotide positions) with 100 pseudo-replicates. Compared to the 28S rDNA
phylogenetic tree in Figure 3, the "temperate clade", only represented by relatively short sequences,
was specifically included here to investigate whether Symbiodinium associated to the ciliate Tiarina
(sequences in bold) belongs to this clade.
Figure S4. Mapping of the relative abundance of rDNA V9 reads strictly identical to the V9 sequence
of the ciliate Tiarina fusus in Tara Oceans stations (surface samples and 20-180 µm size fraction). The
color gradient indicates the relative abundance of V9 reads from green to red for low to high values,
respectively. Black dots indicate that no V9 reads have been detected in the station.
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