RUI: Chemical Defense by Marine Phytoplankton against Protozoan Grazers

化学威慑一直被认为是陆生植物防御食草动物的重要策略。海洋多细胞生物的化学防御也被广泛记载。相比之下，我们对浮游微生物的化学防御知之甚少。拟议中的研究重点是海洋浮游植物的化学防御，它们负责地球上大约一半的光合作用，对抗原生动物食草动物，它们的主要消费者。众所周知，由浮游植物产生的强力毒素，如由甲藻合成的蛤蚌毒素，可以杀死或使捕食者丧失能力。我们认为，不太严重的化学相互作用（仅阻止对特定猎物的捕食）可能更常见，并且可能强烈影响小型浮游生物中捕食者-猎物的相互作用。我们提出了一个最近发现的由球石藻埃米利亚赫胥黎亚急性化学防御模型。在该物种中，原生动物的放牧激活了二甲基磺酰丙酸（DMSP）裂解酶的酶切作用，形成二甲基硫化物（DMS）和丙烯酸酯，并且至少有一种原生动物优先以裂解酶活性低的赫胥利大肠杆菌为食。由于DIVIS和丙烯酸酯的生产在开花形成的藻类分类群中很普遍，我们建议在上述观察的基础上了解这种反应影响原生动物食草性的机制。我们还将详细研究这种反应是如何传达“不要吃我”的化学信息的。利用这个防御模型，我们将开始评估猎物化学如何改变捕食者的选择和摄食动态，以及这些反应如何反过来影响猎物种群的进化。

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.