Structure, partial elemental composition, and size of Thiopedia rosea cells and platelets.

The phototrophic purple sulfur bacterium Thiopedia rosea forms multicellular, gas-vacuolate, regular, flat aggregates (platelets, sheets) held together by slime. Platelets found in eutrophic water consisted of slime (85% of the total wet volume) and 16 cells, while the gas-filled vacuole occupied 44% of the volume of a single wet cell. Individual platelet cells contained central spindle-shaped gas vesicles (which together constitute the cell's gas vacuole), intracytoplasmic membrane vesicles (chromatophores), and peripheral sulfur globules. Cells were surrounded by a Gram-negative type cell envelope and were connected to neighboring cells of the same platelet by mostly unstructured slime. Cells contained detectable amounts of magnesium, phosphorus, sulfur, and potassium as determined by wavelength-dispersive X-ray microanalysis. The large size and relatively low slime density of the platelet, as well as the flat shape, could greatly decrease platelet sedimentation and so stabilize the position of T. rosea within its water column.