(sigh-ann-id-ee-um) is a small coccoid RED ALGA. Although its colour is exactly the same as that of many prokaryotic blue-green algae, careful scrutiny of the cells shows that the pigments is not distributed throughout the cell, but is within a compartment (filling most of the cell). In nature, these cells form a coating several millimeters thick over the substrate. The cells also have an external organic wall. Most cells are solitary but small groups of dividing cells occur. Differential interference contrast. Material from Nymph Creek , a thermal site within Yellowstone National Park, photograph by Kathy Sheehan and David Patterson.