META-DDA: METabolic Activities of Diatom-Diazotroph Associations

浮游植物是生活在海洋表面的光合作用微生物，构成海洋食物网的基础，并推动碳和氮等元素的全球循环。为了在营养物质有限的地区生存，一些浮游植物进化出了共生关系。在许多情况下，共生如何影响每个伙伴的生存，或它们对生态系统功能和养分循环的影响仍不清楚。这个项目主要研究两种浮游植物之间的共生关系--单细胞真核生物硅藻和单细胞固氮蓝藻重氮藻。这些硅藻-重氮菌群落具有广泛的地理分布，通过固定氮气向生物圈提供生物可利用的氮，影响海洋食物网，并影响碳和氮的循环。尽管这些共生体很重要，但人们对它们的基本生理和新陈代谢知之甚少，因为研究人员很难在实验室中培养DDA。在这个项目中，罗德岛大学的一组研究人员正在应用他们开发的在实验室中培养DDA的方法，并就温度和营养对DDA细胞新陈代谢的影响进行实验。新产生的实验室数据正在为开发DDA细胞功能的计算机模型提供信息，该模型嵌入一个简单的生态系统模型，以测试不同的氮源和温度如何影响DDA生态和生态系统功能。该项目正在解决基本的知识差距，从而加强对DDA地理分布和活动的了解，无论是在今天的海洋还是在温度和营养场发生变化的未来海洋。这项研究的更广泛影响包括向海洋学社区提供DDA文化、研究生培训、计算模型分发以及与更广泛社区的联系。该项目支持的研究生正在接受实验和建模方法的交叉培训。与更广泛的社区的接触包括每年在实验室接待一名高中生实习生，并为普通公众和K-12学生制作教育视频。总体而言，这些活动旨在扩大公众对DDA这一全球重大共生关系的理解。该项目正在研究硅藻-重氮菌群落(DDA)的细胞代谢和生理，并通过解决以下关键的长期存在的问题来评估它们的生态系统和生物地球化学影响：1)半毛藻对不同氮源的细胞反应？2)DDA的热生态位如何影响固氮、营养化学计量和地理分布？3)DDA的生理和代谢与共生的硅藻有何不同，以及共生对生态系统水平的影响是什么？由于缺乏培养数据，主要的生态模型假设DDA从氮气中获得100%的氮，尽管有有趣的实验证据表明并非如此。如果DDA生理受到不同氮源的影响，现代生态系统模型中的关键假设将被改变，从而加深我们对DDA在生态系统和生物地球化学功能中所起作用的理解。除了氮源，温度是细胞代谢的重要调节因素，也是生态系统模型中的关键变量。研究小组正在研究共生中每个伙伴在设定DDA热生态位方面的作用，并通过在现在和未来的海洋中进行建模来检查生态系统层面的影响。最后，内共生菌对宿主基因组、转录组和由此产生的生理学的影响实际上是未知的。DDAS与共生硅藻的比较为研究寄主的代谢变化提供了新的见解，并提供了对DDAS作为共生的新的理解。解决这三个问题有助于从根本上理解这种广泛分布的共生关系的影响。该奖项反映了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。

Phytoplankton are photosynthetic microbes that inhabit the surface ocean, form the base of marine food webs, and drive the global cycling of elements like carbon and nitrogen. To survive in regions with limiting nutrients for growth, some phytoplankton have evolved symbiotic relationships. In many cases, it remains unknown how the symbioses influence the survival of each partner or their impacts on ecosystem function and cycling of nutrients. This project focuses on the symbiotic relationship between two phytoplankton -- a single-celled eukaryote diatom and a single-celled nitrogen fixing cyanobacteria called a diazotroph. These diatom-diazotroph associations (DDAs) have broad geographic distributions, provide bioavailable nitrogen to the biosphere via the fixation of nitrogen gas, affect marine food webs, and influence the cycling of carbon and nitrogen. Despite the importance of these symbioses, little is known about their basic physiology and metabolism because it has been difficult for researchers to grow DDAs in the laboratory. In this project, a team of investigators from the University of Rhode Island is applying a method they developed to grow DDAs in the laboratory and conducting experiments on the effects of temperature and nutrients on DDA cellular metabolism. The newly-generated laboratory data is informing the development of a computer model of DDA cellular functioning that embedded in a simple ecosystem model to test how different nitrogen sources and temperatures influence DDA ecology and ecosystem function. The project is addressing fundamental knowledge gaps, leading to an enhanced understanding of DDA geographic distributions and activities both in today's ocean and in a future ocean with altered temperature and nutrient fields. Broader impacts of this study include the provision of DDA cultures to the oceanographic community, graduate student training, computational model distribution, and outreach to the broader community. Graduate students supported by the project are being cross-trained in experimental and modeling approaches. Outreach to the broader community includes hosting a high school student intern in the lab each year and the development of educational videos for the general public and K-12 students. Collectively, these activities are designed to broaden the public understanding of DDAs, a globally significant symbiosis. This project is examining the cellular metabolism and physiology of diatom-diazotroph associations (DDAs) and evaluating their ecosystem and biogeochemical impacts by addressing the following critical, longstanding questions: 1) What is the cellular response of the Hemiaulus DDA to different nitrogen sources? 2) How does the thermal niche of the DDA influence nitrogen fixation, nutrient stoichiometries, and geographic distribution? 3) How does DDA physiology and metabolism differ from asymbiotic diatoms, and what are the ecosystem-level impacts of the symbiosis? Due to a scarcity of culture data, major ecological models assume DDAs gain 100% of their nitrogen from N2, although there is intriguing experimental evidence suggesting otherwise. If DDA physiology is affected by different N sources, key assumptions in modern ecosystem models will be altered, refining our understanding of the role DDAs play in ecosystem and biogeochemical functioning. In addition to N sources, temperature is an important regulator of cellular metabolism and a key variable in ecosystem models. The team of researchers is examining the roles of each partner in the symbiosis in setting the DDA thermal niche and examining ecosystem-level impacts via modeling both in the present day and future oceans. Finally, the impacts of the endosymbiont on the host genome, transcriptome, and resulting physiology are practically unknown. Comparison of DDAs with asymbiotic diatoms is providing new insights into the metabolic modifications of the host and providing new understanding of DDAs as a symbiosis. Addressing these three questions advances fundamental understanding of the impact of this widely-distributed symbiosis.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.