Defence function of pigment granules in Stentor coeruleus

Pigment granules in Stenter coeruleus are extrusive organelles containing t he pigment, stentorin, which provides the blue-green colouration to this ci liate. We studied the defence function of these organelles by 1) observing the interaction between S. coeruleus and the predatory ciliate Dileptus mar garitifer; 2) comparing normally-pigmented cells and artificially-bleached cells of S. coeruleus as prey for the predator; and 3) measuring the toxici ty of chemically-synthesized stentorin to D. margaritifer, S. coeruleus, an d 7 other ciliates. When a Dileptus attacked a Stenter the Stenter released a mass of bluish material and the Dileptus retreated. Bleached cells of S. coeruleus were more vulnerable than normally pigmented cells to the predat or. Stentorin was highly toxic to D. margaritifer (LD50, 0.6-1.0 mug/mlj in the dark, but much less toxic to S. coeruleus (LD50, 90 mug/ml). Under cer tain conditions, Dileptus was killed by normally pigmented Stenter but not by bleached ones. We conclude that pigment granules of S. coeruleus functio n as organelles of defence against D, margaritifer and that the chemical ba sis of this defence is the pigment stentorin.