Defence function of pigment granules in Stentor coeruleus

Pigment granules in Stentor coeruleus are extrusive organelles containing the pigment, stentorin, which provides the blue-green colouration to this ciliate. We studied the defence function of these organelles by 1) observing the interaction between S. coeruleus and the predatory ciliate Dileptus margaritifer; 2) comparing normally-pigmented cells and artificially-bleached cells of S. coeruleus as prey for the predator; and 3) measuring the toxicity of chemically-synthesized stentorin to D. margaritifer, S. coeruleus, and 7 other ciliates. When a Dileptus attacked a Stentor, the Stentor released a mass of bluish material and the Dileptus retreated. Bleached cells of S. coeruleus were more vulnerable than normally pigmented cells to the predator. Stentorin was highly toxic to D. margaritifer (LD 50, 0.6—1.0 μg/ml) in the dark, but much less toxic to S. coeruleus (LD 50, 90 μg/ml). Under certain conditions, Dileptus was killed by normally pigmented Stentor, but not by bleached ones. We conclude that pigment granules of S. coeruleus function as organelles of defence against D. margaritifer and that the chemical basis of this defence is the pigment stentorin.