(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). The cells also have an external organic wall. Differential interference contrast. Material from Nymph Creek and Nymph Lake, thermal sites within Yellowstone National Park, photograph by Kathy Sheehan and David Patterson.