Fig. 1: Detailed structure of bacteriocytes of Bathymodiolus septemdierum.

(A–D) Transmission electron microscopy images showing openings of the symbiotic chambers at the apical surfaces of bacteriocytes (magenta arrowheads in A and B), linkage among several chambers (green arrowheads in A and B), and the complex passages connecting the chambers and the external environment (yellow arrowheads in C and D). Apical is at the top. (E–I) Tomographic three-dimensional reconstruction of bacteriocyte ultrastructure from thin sections of cell 1 observed by scanning electron microscopy (SEM) as a representative showing that almost all symbiotic chambers are interconnected (E–H), with many openings to the external environment at the apical surface (I). Each group of interconnected chambers is labelled with a different colour (the interlinked chambers encompassing the greatest volume are labelled with magenta). (E–H) Views of a single bacteriocyte from four directions, which are indicated at the bottom left of each panel. Because of the resolution of the serial sections (80 nm), several linkages between chambers may have been missed. The nucleus and cytoplasm are labelled with yellow and white, respectively, and made slightly transparent. (I) Apical surface of the 3D reconstructed bacteriocyte. Magenta spots indicate the openings of the chambers. (J–M) SEM images showing many openings at the apical surface of a bacteriocyte of B. septemdierum. (K–M) Magnified images of the cell surface in the areas indicated by green rectangles in (J). Note that the symbionts are observed through the openings (light-blue arrowhead). Scale bars in (A–D and K–M) 1 µm; (I and J) 5 µm.