Colony formation in Scenedesmus:: grazer-mediated release and chemical features of the infochemical

In the unicellularly growing green alga Scenedesmus acutus, the formation of many-celled coenobia may be induced by an infochemical released by the grazer Daphnia magna. We used a standardized bioassay to obtain information about the release of the infochemical by actively feeding D.magna and its chemical nature. The infochemical could not be extracted from the alga or the grazer by aqueous or more lipophilic solvents. When the release of coenobia-inducing activity by actively feeding D.magna was investigated as a function of the individual's body mass, no increase with increasing individual body mass was observed, indicating that the chemical cue originates from zooplankton's non-digestive metabolism rather than from digestion of alga by the grazer. The infochemical released by D.magna can be characterized as an olefinic low-molecular-weight carboxylic acid. Hydroxy and amino groups can be excluded as moieties of the infochemical essential for biological activity. We present a method to concentrate the infochemical from Daphnia incubation water using lipophilic solid-phase extraction. Subsequent separation by reversed-phase HPLC yielded only one active fraction.