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

水生生态系统中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代谢策略生长和繁殖，这些策略塑造了它们在海洋食物网中的位置。例如，生物学家传统上将异养微生物分为初级生产者（利用光合作用产生新的有机物）或异养生物（吃其他生物产生的有机物，如身体）。然而，许多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掉更小的细胞（包括细菌）来“混合”这两种形式的代谢。此外，许多混合营养生物在代谢上是灵活的：它们可能或多或少地依赖于每种代谢来源，这取决于环境条件。由于光合作用（将碳从大气中取出并将其锁定为有机物）和异养（将有机物呼吸回二氧化碳）控制着海洋食物网是否充当碳汇（从大气中净去除二氧化碳），因此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。该项目通过量化混合营养体在短期和长期时间尺度上改变其对光合作用的依赖程度来促进对混合营养体代谢的理解。该项目测试了混合营养体适应温暖和寒冷的水条件的速度，以及这些适应如何改变它们在碳循环中的作用。研究人员-包括研究生，博士后研究人员和本科学员-将测量实验进化的混合营养生物的生理反应，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，它们通过外展研讨会，年度开放日活动和当地圣巴巴拉自然历史博物馆每周的科学报告与公众分享。为了预测气候系统中生物介导的反馈，我们必须了解海洋浮游生物将如何对未来的海洋条件做出反应。虽然一些研究试图量化浮游植物的潜在进化反应，但对变化条件的影响知之甚少（例如，温度升高）。现有的数据表明，混合营养体可能会调节一个积极的气候反馈回路：当变暖时，混合营养体变得更加异养，从而减少了它们对生物泵的贡献，并增强了有机碳的局部呼吸。变暖还可能导致细胞尺寸缩小，减少下沉通量和上层海洋的碳输出。此外，由于预测的海洋分层的增加预计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在一系列温度条件下以完全析因设计实验性地进化混合营养生物，该设计还操纵光（光合作用）和猎物（异养）的可用性。她量化了进化谱系的碳预算、放牧活动、营养成分和食草动物的适口性，以估计任何观察到的适应对碳循环的影响。具体来说，研究人员询问进化谱系与祖先谱系在耐受改变的热条件的能力方面如何比较，并将适应性的差异与依赖光合作用与异养的变化联系起来。同时，她将混合营养模块纳入全球海洋生态地球化学模型，允许量化混合营养生物与当代或进化的生理特征的影响。这项工作将提供一些已知的第一个数据mixotroph塑料和进化的反应，并允许这些反应的规模，其对上层海洋生物地球化学的潜在影响。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.