Collaborative Research: Biogeochemical significance of the abundant, uncultivated symbiotic cyanobacteria UCYN-A

氮是一种养分，其可获得性限制了生态系统的生长和生产力。氮在大气中以惰性的气态氮气形式极其丰富，但大多数生物不能将氮气还原为生物可利用的形式。在包括农业土壤在内的所有环境中，都有微生物可以通过将氮气还原为生物可利用的形式铵来获得氮气。在辽阔的海洋中，很少有生物具有这种能力，最近发现了一种独特的单细胞蓝藻和单细胞藻类共生关系，这种共生似乎非常广泛，可能具有全球生物地球化学意义。这种共生伙伴关系中的蓝藻具有非常不寻常的新陈代谢和基因组流线型。人们对这种共生现象知之甚少，因为只有通过现代分子生物学技术才能检测到这种共生现象。最近的研究表明，这种共生现象在海洋中非常广泛地传播，蓝藻和藻类宿主中都存在以前未被认识到的多样性。这项研究将研究美国沿海水域这种多样性的环境分布和生物地球化学意义。研究人员将通过在当地高中和斯坦福大学为本科生提供实习计划，以及在虚拟游轮博客上记录他们的实地研究，让公众参与到海洋科学中来。在海洋环境中，固定氮气对固定氮(N)池的贡献没有得到很好的量化，部分原因是对重氮菌的丰度、活动和生理的了解不完全。共生单细胞蓝藻(UCyn-A)是一类特征不佳但具有全球重要性的海洋重氮菌。UCyn-A广泛分布于海洋环境中，与一种微真核裸藻共生。我们现在知道UCyn-A有多种生态型，可能适合于水柱中的特定位置和不同的海洋省份。通常情况下，氮的固定在受海岸影响的非热带水域被认为是不重要的，然而最近的数据表明，存在多个UCyn-A亚类。在海洋N循环中，UCyN-A亚类在海岸/近岸环境中的固定氮的分布和速率是一个主要未知的问题。它在近岸水域的存在可能会改变流域氮源(固定氮气)和汇(反硝化)之间的平衡范式。同样，在确定受沿海影响水域的新产量估算时，需要考虑UCyN-A对氮气的显著固定作用。该项目旨在量化不同的UCyn-A亚类对沿海/近岸N预算的意义。它解决了通过在北美西海岸外进行严格的野外工作来确定沿海水域不同UCyN-A亚类固定氮气的问题。UCyn-A亚类群的时空分布以及固氮速率将通过将大量群落和单个细胞的固氮测量(NanSIMS)与分子分析相结合来确定，以研究这些广泛但稀疏的重氮营养共生体及其宿主。此外，研究人员还将进行实验，以限制光照和营养比(N/P)对UCYN-A氮固定率和裸子植物宿主的碳固定率的影响。他们将通过对南加利福尼亚州海湾(斯克里普斯码头)的一个沿海地点以及加利福尼亚州中部和巴哈半岛之间沿海水域的两个程序邮轮进行季节性抽样来进行这项工作。巡航工作将提供一个机会，在更大的空间尺度上了解UCyn-A/裸子植物协会的时间动态。最后，证据表明存在不明的UCyn-A亚类和宿主，研究人员开发了一种策略来识别和量化它们的时间和空间分布以及它们的氮气固定活动。关于UCyn-A的沿海分布、生态和活动的数据对于更好地了解它们对海洋环境固定氮的贡献至关重要。在这项对受海岸影响的水域的研究中测量的特定群体和体积的氮固定速率，将为未来的建模工作提供数据，这将对限制海洋氮固定输入做出重要贡献。

Nitrogen is a nutrient whose availability limits growth and productivity of ecosystems. Nitrogen is extremely abundant in the atmosphere in the inert form of gaseous N2, but most organisms cannot reduce N2 into a biologically available form. In all environments, including agricultural soils, there are microorganisms that can make available the N from gaseous N2 by reducing it to the biologically available form, ammonium. In the vast expanses of the open ocean, few organisms are known to have this ability, and recently a unique symbiosis between a single-celled cyanobacterium and a single-celled algae was discovered, which appears to be very widely distributed and likely of global biogeochemical significance. The cyanobacterium in this symbiotic partnership has very unusual metabolism and genomic streamlining. Little is known of the symbiosis because it is not detectable except by modern molecular biological techniques. Recent work has shown this symbiosis to be very widely spread through the oceans, and that there is previously unrecognized diversity in both the cyanobacterial and algal hosts. This research will examine the environmental distributions and the biogeochemical significance of this diversity in coastal US waters. The investigators will engage the public in ocean sciences through internship programs at local high schools and for undergraduate students at Stanford, and by documenting their field research in a 'virtual cruise' blog.In the marine environment, the contribution of N2 fixation to the fixed nitrogen (N) pool is poorly quantified, in part due to an incomplete understanding on the abundance, activity, and physiology of diazotrophs. The symbiotic unicellular cyanobacteria (UCYN-A) is a poorly characterized, yet globally important, group of marine diazotrophs. UCYN-A is widely distributed in the marine environment, and lives symbiotically with a picoeukaryotic prymnesiophyte alga. We now know that there are multiple ecotypes of UCYN-A, which may be adapted to specific locations in the water-column and different oceanic provinces. Typically N2 fixation was considered unimportant in coastally influenced and non-tropical waters, however recent data shows that multiple subclades of UCYN-A are present. The distribution and rate of N2 fixation by UCYN-A subclades in coastal/nearshore environments is a major unknown in the oceanic N cycle. Its presence in nearshore waters may change the paradigm of the balance between basin N sources (N2 fixation) and sinks (denitrification). Likewise, significant N2 fixation by UCYN-A will need to be considered when determining estimates of new production in coastally influenced waters. This project aims to quantify the significance of different UCYN-A subclades to coastal/nearshore N budgets. It tackles the issue of determining N2 fixation rates by different UCYN-A subclades in coastal waters through rigorous fieldwork off the west coast of North America. The temporal and spatial distribution of UCYN-A subclades, as well as the rates of N2 fixation, will be determined by coupling N2 fixation measurements of bulk communities and individual cells (nanoSIMS) with molecular assays to study these widespread, but dilute, diazotrophic symbionts and their hosts. Additionally the investigators will conduct experiments aimed at constraining the effects of light and nutrient ratios (N/P) on UCYN-A N2 fixation rates, and the prymnesiophyte host's rate of carbon fixation. They will conduct this work through seasonal sampling of a coastal site in the Southern California Bight (Scripps Pier) and on two process cruises in the coastal waters between central California and the Baja Peninsula. The cruise work will provide an opportunity to understand the temporal dynamics of the UCYN-A/prymnesiophyte associations over larger spatial scales. Finally, evidence suggests that unidentified UCYN-A subclades and hosts exist and the investigators have developed a strategy to identify and quantify their temporal and spatial distributions as well as their N2 fixation activities. Data on the coastal distribution, ecology and activity of UCYN-A is critical for obtaining a better understanding of their contribution to fixed N to the marine environment. The group-specific and bulk rates of N2 fixation measured in this study of coastally influenced waters, will provide data for future modeling efforts, which will make an important contribution to constraining oceanic N2 fixation inputs.