META-DDA: METabolic Activities of Diatom-Diazotroph Associations

浮游植物是栖息在海洋表面的光合微生物，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，共生体如何影响每个伙伴的生存或它们对生态系统功能和营养物质循环的影响仍然是未知的。该项目的重点是两种浮游植物之间的共生关系-单细胞真核硅藻和单细胞固氮蓝藻称为固氮菌。这些固氮生物群落具有广泛的地理分布，通过固定氮气为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，来自罗得岛大学的一组研究人员正在应用他们开发的一种方法在实验室中培养DDA，并就温度和营养物质对DDA细胞代谢的影响进行实验。新生成的实验室数据为DDA细胞功能计算机模型的开发提供了信息，该模型嵌入简单的生态系统模型中，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，从而加深对DDA在当今海洋和未来温度和营养场发生变化的海洋中的地理分布和活动的了解。这项研究的更广泛的影响包括提供DDA文化的海洋学社区，研究生培训，计算模型的分布，并推广到更广泛的社区。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。与更广泛的社区的联系包括每年在实验室接待一名高中生实习生，以及为公众和K-12学生制作教育视频。总的来说，这些活动的目的是扩大公众对DDAs的理解，这是一种具有全球意义的共生关系。本项目通过解决以下关键的、长期存在的问题来研究半沟藻固氮菌群（DDA）的细胞代谢和生理学，并评估其生态系统和生物地球化学影响：1）半沟藻DDA对不同氮源的细胞反应是什么？2)DDA的热生态位如何影响固氮、养分化学计量和地理分布？3)DDA的生理和代谢与非共生硅藻有何不同，共生对生态系统层面有何影响？由于缺乏培养数据，主要的生态模型假设DDAs从N2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生理学受到不同N源的影响，现代生态系统模型中的关键假设将被改变，从而完善我们对DDA在生态系统和生态地球化学功能中所起作用的理解。除氮源外，温度是细胞代谢的重要调节因子，也是生态系统模型中的关键变量。研究小组正在研究每个合作伙伴在共生中的作用，以确定DDA热生态位，并通过对当今和未来海洋的建模来研究生态系统层面的影响。最后，内共生体对宿主基因组、转录组和由此产生的生理学的影响实际上是未知的。将DDAs与非共生硅藻进行比较，为宿主的代谢修饰提供了新的见解，并为DDAs作为共生体提供了新的理解。解决这三个问题推进了对这种广泛分布的共生关系的影响的基本理解。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.