Eating themselves sick? Ecological interactions among a mixotrophic flagellate, its prokaryotic prey, and an ingestible giant virus.

浮游植物为海洋提供丰富的生物资源，因此了解是什么控制着它们的生长和死亡是海洋学的核心问题之一。在大部分营养物质枯竭的开阔海洋表层水中，最成功的浮游植物是被称为原绿球藻的微小光合细菌。这些细菌是海洋有限营养物质的非常成功的竞争者，通常比大型浮游植物更有竞争力。然而，海洋中存在着许多较大种类的浮游植物。这种共存可能发生的一个原因是，一些被称为混合营养体的较弱竞争对手进化出了一种聪明的替代策略，最好的概括是“如果你不能打败它们，就吃掉它们”。除了直接争夺溶解在水中的营养物质外，这些较大的浮游植物还可以通过消耗和消化较小的竞争对手来获取营养物质。光合作用和吃掉竞争对手的双重能力具有明显的优势，但这种内部捕食策略可能存在隐性成本。在以原胆球菌为食的同时，混合营养体也可能无意中摄入巨大的病毒，这些病毒太大了，以至于被误认为是食物。感染通常是致命的。混合营养和病毒感染在海洋中无处不在；然而，传统的海洋食物网模型通常单独考虑光合作用和捕食，而这些过程往往没有得到充分的研究和遗漏。在本项目中，将在实验室和现场研究一种常见的混合营养菌（Florenciella）、它的猎物（原绿球藻）和感染混合营养菌的病毒（FloV1）之间的相互作用。这项研究也将有助于指导内聚性混合营养-病毒-猎物营养模型的发展。改进这些营养模型来解释更复杂的过程可以从根本上改变我们对海洋营养动力学的理解。该项目将直接支持跨学科科学的博士后、研究生和本科生的培训，包括实地、实验室和建模活动。该项目将支持研究生论文的主要组成部分和本科生的渐进式培训，最终形成一个独立的项目。这里研究的混合营养和病毒生态学的概念将转化为数百名儿童和公众可以看到的公共展示。这些pi将通过“科学教师上船研究船（STARS）”计划聘请一名K-12教师在海上进行实地考察，并将通过夏威夷大学的CMORE学者计划招募一名本科研究员。混合营养策略的优点和缺点将取决于资源和竞争者的可用性以及病毒感染的可能性。将测试放牧的时间，以确定Florenciella是连续放牧还是只在夜间进食，将放牧和光合活动分开。Florenciella的猎物偏好将在竞争性放牧实验中进行测试，在不同数量的其他细菌和蓝藻存在的情况下，提供原绿球藻作为猎物。电子显微镜将用于确定猎物和病毒是否通过相同的途径进入弗洛伦斯氏菌，以及猎物的存在是否竞争性地干扰病毒感染。将对弗洛伦斯氏菌放牧和弗洛伦斯氏菌感染的动力学进行定量分析。这些实验室实验的结果将用于参数化数值模型。该模型将用于回答问题并对混合营养-病毒-猎物系统的动力学进行预测，并将这些预测与现场数据进行比较。总的来说，这些观察、实验和定量分析将提供对隐藏在海洋食物网底层的生态复杂性的详细探索。

Phytoplankton support the biological bounty of our seas, so understanding what controls their growth and death is one of the central issues in oceanography. In much of the nutrient-depleted surface waters of the open ocean, the most successful phytoplankton are tiny photosynthetic bacteria known as Prochlorococcus. These bacteria are highly successful competitors for the ocean's limited nutrients and commonly outcompete larger phytoplankton. Yet, many larger types of phytoplankton persist in the ocean. One reason why this coexistence may occur is that some of the weaker competitors called mixotrophs have evolved a clever alternative strategy best summed up as "If you can't beat them, eat them". In addition to directly competing for nutrients dissolved in the water, these larger phytoplankton can acquire nutrients by consuming and digesting their smaller rivals. The dual ability to photosynthesize and eat competitors has clear advantages, but there can be hidden costs of this intraguild predation strategy. While feeding on Procholorococcus, mixotrophs may also inadvertently ingest giant viruses that are so large they are mistaken for food. Infection is often fatal. Mixotrophy and viral infection are ubiquitous in the ocean; however these processes are often understudied and missing from traditional models of marine food webs that generally consider photosynthesis and predation independently. In this project, the interactions among a common mixotroph (Florenciella), its prey (Prochlorococcus), and a virus that infects the mixotroph (FloV1) will be studied in the lab and field. This research will also help guide the development of a cohesive mixotroph-virus-prey trophic model. Improving these trophic models to account for more complex processes could fundamentally change our understanding of marine trophic dynamics. The project will directly support the training of a post-doc, graduate and undergraduate student in inter-disciplinary science that includes field, lab, and modeling activities. The project will support a major component of the graduate student's dissertation and the progressive training of an undergraduate student, culminating in an independent project. The concepts of mixotrophy and viral ecology investigated here will be translated into a public display seen by hundreds of children and members of the public. The PIs will engage a K-12 teacher in the fieldwork at sea through a "Science Teachers Aboard Research Ships (STARS)" program and will recruit an undergraduate researcher through the CMORE Scholars program at the University of Hawaii.The advantages and drawbacks of a mixotrophic strategy will depend on the availability of resources and competitors and the likelihood of viral infection. The timing of grazing will be tested to determine whether Florenciella grazes continuously or separates it grazing and photosynthetic activities by only feeding at night. Prey preferences of Florenciella will be tested in competitive grazing experiments offering Prochlorococcus as prey in the presence of varying amounts of other bacteria and cyanobacteria. Electron microscopy will be used to determine whether prey and virus enter Florenciella by the same pathway and whether the presence of prey competitively interferes with viral infection. The kinetics of grazing by Florenciella and infection of Florenciella by FloV1 will be quantified. The results from these lab experiments will be used to parameterize a numerical model. The model will be used to answer questions and make predictions about the dynamics of the mixotroph-virus-prey system and those predictions will be compared to field data. Collectivity, these observational, experimental and quantitative analyses will provide a detailed exploration of the ecological complexity hidden at the base of the marine food web.