Kairomone-induced colony formation in freshwater phytoplankton: Algal population dynamics and physiological cost of coloniality

Autotrophs comprise the base of the food web and support heterotrophic organisms. Morphological plasticity in autotrophs is often adaptive and in many cases provides defense against certain herbivores or grazers, which in turn can alter the herbivore community and consequently the structure of the entire food web. Studies have shown that chemical signals from herbivores can induce anti-herbivore defense in autotrophs, as in the Daphnia - Scenedesmus/Desmodesmus system. Green algae Scenedesmus and Desmodesmus form colonies in response to a chemical cue (a kairomone) released from Daphnia (a planktonic grazer). Colony formation reduces algal mortality by interfering with filter feeding by the grazers. This thesis investigated the relationship between kairomone-induced colony formation and population dynamics of Scenedesmus and Desmodesmus in order to reveal possible physiological tradeoffs involved in the anti-grazer mechanism. A negative relationship was observed between the colony-forming effect of the kairomone and algal growth rate (Chapters 1 and 2), which suggested an increased benefit of colony formation at low algal growth rates. Utility of a synthetic kairomone substitute, octyl sodium sulfate (OSS) was tested and supported (Chapter 1). Interactions between nutrient types and availabilities and the kairomone effect were studied in detail, and organic phosphorus and nitrogen were found to be the key factors controlling unicell production in Desmodesmus (Chapter 2). Finally, tradeoffs were examined in experiments where unicellular and colonial Desmodesmus competed against another green alga, Monoraphidium (Chapter 3). The ability of Desmodesmus to outcompete Monoraphidium was lessened by kairomone-induced colony formation, indicating a physiological cost for the alga that trades off against the inducible anti-grazer defense. The findings in this thesis connect for the first time previous organismal-level studies on algal colony formation mechanisms to population-level studies on evolutionary tradeoffs and population interaction.