CHEMICAL INDUCTION OF COLONY FORMATION IN A GREEN-ALGA SSCENEDESMUS-ACUTUS) BY GRAZERS (DAPHNIA)

The green alga, Scenedesmus acutus, grows in culture in unicellular fo rm, but it forms colonies (coenobia) when exposed for 48 h to a chemic al released by the grazer Daphnia magna. The colony-forming response c an be evoked only in growing cells. The Daphnia factor affects colony size but not algal growth rate. The minimum concentration of Daphnia f actor that induces colony formation is equivalent to a Daphnia biomass of 0.5 mg dry wt liter(-1) in the culture medium. Actively feeding da phniids induce a stronger response than starved ones. Homogenized Scen edesmus, homogenized Daphnia, ammonium, and urea are not effective. Th e Daphnia factor is a nonvolatile, organic substance of small molecula r mass (<500 Da). It is moderately lipophilic, heat stable, pH-resista nt in a range from 1 to 12, and not affected by treatment with Pronase E. The chemical activity is not lost when the substance is dried but disappears during incineration. Colony formation can be interpreted as a grazing defense mechanism. The phenotypic response may have evolved because of the tradeoff between higher sinking rates and grazing resi stance of colonial forms.