Chemical induction of colony formation in a green alga (Scenedesmus acutus) by grazers (Daphnia)

摘要: The green alga, Scenedesmus acutus, grows in culture unicellular form, but it forms colonies (coenobia) when exposed for 48 h to a chemical released by the grazer Daphnia magna. colony-forming response can be evoked only growing cells. factor affects colony size not algal growth rate. minimum concentration of that induces formation is equivalent biomass 0.5 mg dry wt liter-* medium. Actively feeding daphniids induce stronger than starved ones. Homogenized Scenedesmus, homogenized Daphnia, ammonium, and urea are effective. nonvolatile, organic substance small molecular mass (< 500 Da). It moderately lipophilic, heat stable, pH-resistant range from 1 12, affected treatment with Pronase E. activity lost dried disappears during incineration. Colony interpreted as grazing defense mechanism. phenotypic may have evolved because tradeoff between higher sinking rates resistance colonial forms. Phytoplankton extremely variable, species composed many different clones replace each other under differing environmental conditions (Wood Leatham 1992). Clones established single cells also change their morphology grown laboratory conditions; thus, they phenotypically plastic. genus known notoriously flexible (Trainor 199 1). Individual strains various grow unicells or form four eight vary respect number spines. Trainor (1992) claimed an ordered sequence ecomorphs defined cyclomorphosis (sensu Black Slobodkin 1987) driven factors. Various abiotic factors (nutrients, pH) age affect (Egan 1989), temperature particularly effective controlling phenotypes. Unicells predominant at warm temperatures,