Figure 1. A bundle of Thioploca filaments from marine sediments close to the Peruvian coast. The bundle is placed next to a ruler of 5 cm in length. The Thioploca filaments appear white due to internal sulfur granules that reflect the light.

Figure 2. Large intestinal symbionts of surgeonfish. These panels, all shown at the same magnification, illustrate the morphological diversity of ‘Epulopiscium’ and its relatives. The cell on the left is the second largest morphotype, Epulopiscium type B. Visible are two large internal offspring cells. Other rod-shaped and filamentous forms are shown. Scale bar represents 20 m. Angert, ER (2006) The enigmatic cytoarchitecture of Epulopiscium spp., Microbiology Monographs, Complex Intracellular Structures in Prokaryotes, vol. 2 p. 288. Berlin Heidelberg, Germany: Springer-Verlag. Reproduced with kind permission of Springer Science and Business Media.

Figure 3. Examples of large-celled cyanobacteria. All scale bars are 10 m. (a) Chroococcus sp., dividing (left) and recently divided cells from this typically freshwater genus. (b) Porphirosiphon notarissi, a filamentous cyanobacterium allied to the large Oscillatoria, isolated from a soil crust in the Namibian desert, displaying the typical disc-shaped cells in two separate trichomes, enclosed in a common extracellular sheath. Note the apoptotic, vestigial cell between the two trichomes that served to separate them in a process of self immolation. (c) Detail of a morphologically complex Stigonema spp. displaying a main filament consisting of two series of cells, and an incipient uniseriate branch.

Figure 4. The morphology of the giant sulfur bacteria as they appear in the light microscope: (a) Beggiatoa, (b) Thioploca, (c) Thiothrix, (d) Thiomargarita, (e) Thiovulum, (f) Achromatium. In a–e numerous sulfur inclusions can be seen. Achromatium (f) is the only known bacterium with calcite inclusions. The much larger calcite inclusions conceal the sulfur inclusions, which are also present. The scale bars show typical sizes for each genus. Nevertheless, the cell diameters of sulfur bacteria are highly variable.