Colony formation in Scenedesmus: No contribution of urea in induction by a lipophilic Daphnia exudate

The common green alga Scenedesmus may respond morphologically to numerous environmental factors. The formation of colonies in Scenedesmus resulting from exposure to grazer (Daphnia) excreta is of particular interest since the induced colony formation may be an induced defense. Recent studies suggested that unicell-colony transformation in Scenedesmus could result from urea released by actively feeding Daphnia and/or by a more lipophilic substance that could be retained by lipophilic solid phase extraction (C18-SPE). We investigated the generality of the contribution of urea to the overall morphogenetic activity by separating urea and the more lipophilic infochemical using C18-SPE. No morphogenetic effect of urea was detected in two different algal growth media, whereas Daphnia water induced colony formation. After C18-SPE of active Daphnia water, the urea-containing run-through was inactive, whereas the desorbed eluate remained active. Thus, colony-inducing activity is more lipophilic than urea-related