Collaborative Research: Did the SE Pacific Gyre become a Hot Spot for N2 Fixation during Dusty Glacial Conditions?

氮元素是所有生物的基本组成部分，它在环境中的循环是地球生物圈的重要组成部分。作为一种重要的营养物质，生物可用形式的氮的可用性往往“限制”了陆地和海洋植物的生长。矛盾的是，氮以二氮气体（N2）的形式在地球大气和海水中都非常丰富。然而，在这种化学形式下，氮不能被大多数生物利用。只有一小部分微生物有能力“固定”N2气体，也就是说，将其转化为生物上可用的化学形式。因此，这些固氮生物提供了维持地球生命的关键环境功能。在海洋中，固氮作用是控制生物可利用氮总量的主要因素，平衡了时间损失回氮气的过程。生物可利用氮的数量反过来控制海洋表面阳光照射区域光合生物（浮游植物）的生长（生产力），这些生物构成食物链的基础，并有助于海洋控制大气中温室气体的水平。该项目关注的是了解海洋固氮的基本、大规模控制，以及它们如何受到气候变化的影响。固氮微生物本身似乎受到其他营养元素如磷和铁的可用性的限制。虽然我们知道目前在海洋的哪些部分有更多或更少的磷和铁的可用性，但尚不清楚两者是否具有压倒一切的重要性，或者过去的变化是否在固氮方面产生了重大变化。过去氮固定的变化可能是海洋控制大气温室气体的重要反馈。了解这些过去的变化及其控制因素将为更好地预测海洋氮固定如何响应未来气候变化提供知识基础。这具有重大的社会意义，因为海洋氮固定的变化将最终影响海洋生态系统和生物资源，作为温室气体驱动气候变化的反馈。为了解决这些问题，研究小组将进行一项研究，研究东南太平洋环流在最后一个冰期周期内对N2固定的气候敏感性，以及其似是而非的“主控因素”。尽管邻近的秘鲁-智利氧最小带提供了大量过剩的磷酸盐，但这个营养少的区域几乎没有现代氮固定。这与现代铁的限制是一致的，这是由于在过去的多尘条件下，低风沙供应将得到缓解。研究小组将利用最后一个完整冰期循环的自然实验，在环流点及其南缘的有孔虫结合的N同位素中捕获，来探测由不同的尘埃输入和相邻最小氧区N损失的变化对海洋N循环的控制，并将这些与已知的温室气候强迫变化联系起来。通过数值模拟，研究小组还将考虑该地区过去的N2固定变化是否影响了全球海洋N循环和收支。该项目还将资助本科生和研究生的培训，并支持来自代表性不足群体的高中生参与原创性研究。研究小组将通过与小学、初中和高中建立伙伴关系，继续他们的推广工作，让不同的学校人口和他们的家庭参与令人兴奋和相关的科学。

The element nitrogen is a fundamental component of all living things and its cycling through the environment is an important component of Earth's biosphere. As a vital nutrient, the availability of nitrogen in a biologically usable form often "limits" the growth of plants both on land as well as in the ocean. Paradoxically, nitrogen is very abundant as dinitrogen gas (N2) in both the Earth's atmosphere and dissolved in seawater. However in this chemical form, nitrogen cannot be used by most living things. Only a small subset of microbes has the ability to "fix" N2 gas, that is, to convert it into a biologically usable chemical form. Thus, these N2 fixing organisms provide a critical environmental function sustaining life on this planet. In the ocean, N2 fixation is a major control on the total amount of biologically available nitrogen, balancing over the time losses back to N2 gas. The amount of biologically available nitrogen in turn controls the growth (productivity) of photosynthetic organisms (phytoplankton) in the sunlit region of the surface ocean which form the base of the food chain and contribute to oceanic control of the atmospheric levels of greenhouse gases. This project concerns itself with understanding the fundamental, large-scale controls of oceanic N2 fixation and how they are influenced by climate change over time. N2 fixing microbes themselves appeared to be limited by the availability of other nutrient elements such as phosphorous and iron. While it is known in which parts of the ocean there is at present greater or lesser availability of phosphorous and iron, it remains unclear if either is of overriding importance or if changes in the past produced significant variations in N2 fixation. Past changes in N2 fixation may have been an important feedback on oceanic control of atmospheric greenhouse gases. Understanding these past changes and their controls will provide the knowledge base for improving prediction of how ocean N2 fixation may respond to future changes in climate. This is of great societal relevance as changes in oceanic N2 fixation will ultimately impact marine ecosystems and living resources as feedback on the greenhouse gases driving climate change.To address these questions, the research team will undertake a study the climate-sensitivity of N2 fixation in the southeast Pacific gyre over the last glacial cycle as well as its plausible "master controls". This oligotrophic region experiences little modern N2 fixation despite proximity to a large supply of excess phosphate from the adjacent Peru-Chile oxygen minimum zone. This is consistent with modern iron limitation due to low aeolian supply that would have been relieved during past dusty conditions. The research team will use the natural experiment of the last full glacial cycle, captured in the foraminiferal-bound N isotopes of gyre sites as well as sites at its southern margin, to probe controls on the marine N cycle exerted by variable dust inputs and changes in N-loss in the adjacent oxygen minimum zone, and relate these to known changes in greenhouse forcing of climate. Through numerical modeling, the research team will also consider whether past variations in N2 fixation in this region may have impacted the global ocean N cycle and budget. This project will also fund the training of undergraduate and graduate students and support participation of high school students from underrepresented groups in original research. The research team will continue their outreach efforts through established partnerships with elementary, middle, and high schools, engaging a diverse school population and their families with exciting and relevant science.