Fig. 1. Morphology and ultrastructure of the magnetotactic strain WYHC-5.

(A) Optical micrograph showing living coccoidal MTB (WYHC-5). They swim out from the sediment located to the left of the image (sediment not shown) along the external applied magnetic field lines (dashed line with arrow) and gather on the water droplet edge. A few other types of small MTB are indicated by yellow arrows. (B) Optical micrograph of WYHC-5 cells magnetically collected from sediments in laboratory microcosms. The bacteria were stained by crystal violet before optical microscopy observations. Two out-of-focus cells are indicated by yellow arrows. (C) STEM image of one typical WYHC-5 cell acquired in the STEM-HAADF showing multiple chains of magnetosome magnetite (Mag), numerous electron-dense inclusions (Inc), and electron-transparent vacuoles (Vac). (D) SEM image of air-dried WYHC-5 cell selected from a series of FIB sections showing that the vacuoles, magnetosome magnetite, and electron-dense inclusions are relatively independent from each other within the cell. The empty vacuoles remain approximately spherical in shape, suggesting that they have a rigid enclosed structure. (E) STEM-HAADF image of an ultrathin section of a freeze-substituted WYHC-5 cell showing (i) the typical Gram-negative cell wall (CM, cytoplasmic membrane; OM, outer membrane; PG, peptidoglycan) and (ii) vacuoles adjacent but separated from the CM. (F) Bright-field TEM image of an ultrathin section of a freeze-substituted WYHC-5 cell showing empty magnetosome membrane (MM; the inside magnetite crystal might have been lost during sample preparation), and the aggregate of polyhedral carboxysome-like bodies (Carl). (G) 3D FIB-SEM reconstruction and visualization of one freeze-substituted WYHC-5 cells [green, magnetosome magnetite; gray, vacuole; yellow, carboxysome-like body; light blue, cytoplasm (Cyt)]. The FIB-SEM sections and 3D reconstructions for air-dried and freeze-substituted WYHC-5 cells are provided as movies S2 and S3, respectively.