Chemical deterrence has long been recognized as an important strategy by which terrestrial macrophytes defend themselves against herbivores. Chemical defense also has been documented extensively for marine multicellular organisms. By comparison, we know astonishingly little about chemical defenses by planktonic microbes. The proposed study focuses on chemical defense by marine phytoplankton, which are responsible for roughly half the photosynthesis on Earth, against protozoan grazers, their dominant consumers. It is well known that potent phytoplankton-derived toxins, such as the saxitoxin synthesized by the dinoflagellate Alexandriwn, can kill or incapacitate predators. We argue that less severe chemical interactions, which merely deter feeding on specific prey, may be more common and may strongly influence predator-prey interactions within the microplankton. We present a recently-discovered model for subacute chemical defense by the coccolithophorid alga Emilianiahuxleyi. In this species, enzymatic cleavage by dimethylsulfoniopropionate (DMSP) lyase to form dimethyl sulfide (DMS) and acrylate is activated by protozoan grazing, and at least one protozoan species feeds preferentially on E. huxleyi clones with low lyase activity. Since production of DIVIS and acrylate is widespread among bloom-forming algal taxa, we propose to build on the above observations to understand the mechanisms by which this reaction influences protozoan herbivory. We will also investigate in detail how the reaction serves to communicate the chemical message `don't feed on me`. Using this defense model, we will begin to evaluate how prey chemistry alters predator choice and feeding dynamics, and how these responses might in turn affect the evolution of prey populations.
