Extended Data Fig. 4: Microscopic characterization of ‘Candidatus Tectiglobus diatomicola’–Haslea symbioses.

a–d, Confocal laser scanning microscopy images (a,b) and corresponding scanning electron micrographs (c,d) of dividing diatoms with six (a,c) and eight (b,d) ‘Ca. T. diatomicola’ cells, after hybridization with oligonucleotide probes (in pink, overlay of Hypho1147 and Hypho734 in red and blue, respectively; see Extended Data Table 2) and counterstaining with DAPI (white). Solid and dashed lines indicate assumed outline of the dividing diatoms. e, Confocal laser scanning microscopy image of a Haslea diatom after CARD-FISH with the NON338-probe showing no unspecific binding at the typical symbiont location (autofluorescence of the chloroplasts at 488 nm in green, NON338-probe background signal in light blue, DAPI in white). f, Epifluorescence image of the ‘Ca. T. diatomicola’–Haslea symbiosis, showing ‘Ca. T. diatomicola’ cells after hybridization with a specific oligonucleotide probe (Hypho638 in blue), autofluorescence of the chloroplasts in green and the H-shaped nucleus counterstained with DAPI (white). The bilobed chloroplasts are located on either side of the central valve area, and the lobes of the chloroplasts are connected along the cell’s transapical axis. g–k, Scanning electron micrographs of the diatom host. g, Whole frustule showing the internal raphe structure and an apparent hyaline area around the central raphe region (arrow) as well as sections of the hyaline girdle bands (arrowheads). h, Valve end showing the ridges bordering the central raphe. i, Central valve area. The valve’s normal surface structure of longitudinal ribs overlying the transapical striae appears modified with the longitudinal ribs damaged and only the transapical structures visible. j, Valve pole showing the shape of raphe end (arrow head). k, Central valve area with the central valve area collapsed. Arrowheads point to a chamber-like structure. Scale bars are 5 µm in a–g and 1 µm in h–k.