BEE: Testing the evolutionary responses of mixotrophs to future ocean conditions

水生生态系统拥有各种各样的单细胞微生物。这些物种中的许多生活在水面附近，在那里它们使用不同的新陈代谢策略生长和繁殖，这些策略决定了它们在海洋食物网中的位置。例如，生物学家传统上将浮游微生物分为初级生产者(利用光合作用创造新的有机物质)或异养微生物(以其他生物产生的有机物为食)。然而，大量物种实际上是混合营养的：它们通过同时进行光合作用和吃更小的细胞(包括细菌)来“混合”这两种新陈代谢形式。此外，许多混合营养菌在代谢上是灵活的：它们可能或多或少依赖于每一种代谢来源，这取决于环境条件。由于光合作用(将碳从大气中带走并将其锁定在有机物质中)和异养作用(将有机物质呼吸回二氧化碳)控制着海洋食物网是否起到碳汇的作用(从大气中净去除二氧化碳)，了解混合营养生物的新陈代谢对于预测海洋浮游生物对大气碳的影响至关重要。这个项目通过量化混合营养生物在短时间和长时间尺度上改变其对光合作用的依赖的程度，促进了对混合营养物质代谢的理解。该项目测试了混合营养生物适应较暖和较冷水条件的速度，以及这些适应如何改变它们在碳循环中的作用。包括研究生、博士后研究员和本科生在内的研究人员将测量实验进化的混合营养体的生理反应，并使用数学模型将这些变化与全球海洋碳循环联系起来。随着数据的收集，这些数据通过外展研讨会、年度开放参观活动和每周在当地圣巴巴拉自然历史博物馆举行的科学演示与公众分享。为了预测气候系统中的生物中介反馈，我们必须了解海洋浮游生物将如何应对未来的海洋状况。虽然一些研究试图量化浮游植物的潜在进化反应，但关于变化条件(例如，温度升高)对混合营养生物的影响知之甚少。现有的数据表明，混合营养生物可能调节一个正的气候反馈循环：当变暖时，混合营养生物变得更加异养，从而减少它们对生物泵的贡献，并增强有机碳的局部呼吸。气候变暖还可能导致气泡尺寸变小，减少下沉通量和上层海洋的碳出口。此外，由于预计海洋层化的增加将有利于混合营养生物，它们的代谢反应可能对理解全球碳循环越来越重要。该项目的PI是在一系列温度条件下以完全析因设计的试验性进化混合营养生物，该设计还操纵光(光合作用)和猎物(异质营养)的可用性。她量化了进化世系的碳收支、放牧活动、营养含量和食草动物适口性，以估计任何观察到的适应对碳循环的影响。具体地说，研究人员询问进化的谱系与祖先谱系在耐受炎热条件变化的能力方面的比较，并将适应能力的差异与依赖光合作用而不是异质营养的变化联系起来。同时，她将混合营养模块纳入全球海洋生物地球化学模型，使具有当代或进化生理特征的混合营养生物的影响得以量化。这项工作将提供一些关于混养生物塑料和进化反应的第一批已知数据，并允许根据它们对上层海洋生物地球化学的潜在影响来调整这些反应。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Aquatic ecosystems host a wide variety of single-celled, microscopic organisms. Many of these species live near the surface of the water, where they grow and reproduce using different metabolic strategies that shape their place in the marine food web. For example, biologists have traditionally grouped planktonic microbes into either primary producers (which use photosynthesis to create new organic matter) or heterotrophs (which eat organic matter - such as the bodies - produced by other organisms). However, a large number of species are actually mixotrophic: they "mix" these two forms of metabolism by simultaneously conducting photosynthesis and eating smaller cells, including bacteria. Furthermore, many mixotrophs are metabolically flexible: they may rely more or less on each source of metabolism depending on environmental conditions. Because photosynthesis (which takes carbon out of the atmosphere and locks it into organic matter) and heterotrophy (which respires organic matter back into carbon dioxide) control whether or not oceanic food webs act as carbon sinks (having a net removal of carbon dioxide from the atmosphere), understanding mixotroph metabolism is critical to predicting the effects of marine plankton on atmospheric carbon. This project advances understanding of mixotroph metabolism by quantifying the extent to which mixotrophs can alter their reliance on photosynthesis over short and long timescales. The project tests how quickly mixotrophs can adapt to both warmer and colder water conditions, and how these adaptations alter their role in the carbon cycle. Researchers - including graduate students, a postdoctoral researcher, and undergraduate trainees - will measure the physiological responses of experimentally evolved mixotrophs and use mathematical models to connect these changes to global oceanic carbon cycling. As data are collected, they are shared with the public through outreach seminars, annual open house events, and weekly scientific presentations at the local Santa Barbara Museum of Natural History.In order to predict biologically mediated feedbacks in the climate system, we must understand how marine plankton will respond to future ocean conditions. While a number of studies have sought to quantify the potential evolutionary response of phytoplankton, much less is known about the impacts of shifting conditions (e.g., increased temperature) on mixotrophs. What data are available suggest that mixotrophs may modulate a positive climate feedback loop: when warmed, mixotrophs become more heterotrophic, thus reducing their contribution to the biological pump and enhancing local respiration of organic carbon. Warming may also result in reductions in cell size, reducing sinking fluxes and carbon export from the upper ocean. Furthermore, because the predicted increase in oceanic stratification is expected to favor mixotrophs, their metabolic responses may be increasingly significant to understanding the global carbon cycle. The PI of this project is experimentally evolving mixotrophs under a range of temperature conditions in a fully factorial design that also manipulates the availability of light (photosynthesis) and prey (heterotrophy). She quantifies the carbon budget, grazing activity, nutrient content, and grazer palatability of evolved lineages in order to estimate the impact of any observed adaptations on carbon cycling. Specifically, the investigator asks how evolved lineages compare to ancestral lineages in their ability to tolerate altered thermal conditions, and connects differences in fitness to shifts in reliance on photosynthesis versus heterotrophy. Simultaneously, she incorporates a mixotrophy module into a global ocean biogeochemistry model, allowing the quantification of the impact of mixotrophs with either contemporary or evolved physiological traits. This work will provide some of the first known data on mixotroph plastic and evolutionary responses, and allow the scaling of these responses to their potential impacts on upper ocean biogeochemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.