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

氮是一种营养物质，其可用性限制了生态系统的生长和生产力。氮在大气中以惰性气体N2的形式存在，但大多数生物不能将N2还原成生物可利用的形式。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气态氮气还原为生物可利用的形式——铵，从而使氮成为可利用的氮。在广阔的开放海洋中，很少有生物知道有这种能力，最近发现了一种独特的单细胞蓝藻和单细胞藻类之间的共生关系，这种共生关系似乎分布非常广泛，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。人们对这种共生关系所知甚少，因为除非用现代分子生物学技术，否则无法检测到它。最近的研究表明，这种共生关系在海洋中非常广泛地传播，并且在蓝藻和藻类宿主中都存在以前未被认识到的多样性。本研究将研究这种多样性在美国沿海水域的环境分布和生物地球化学意义。调查人员将通过在当地高中和斯坦福大学本科生的实习项目，以及在“虚拟巡航”博客上记录他们的实地研究，让公众参与海洋科学。在海洋环境中，由于对重氮营养体的丰度、活性和生理的不完全了解，对固定氮(N)库中N2固定的贡献的定量很少。共生单细胞蓝藻（UCYN-A）是一种特征较差，但全球重要的海洋重氮营养菌群。UCYN-A广泛分布于海洋环境中，与一种微真核原核藻类共生。我们现在知道，UCYN-A有多种生态类型，它们可能适应于水柱中的特定位置和不同的海洋省份。通常认为，在受沿海影响和非热带水域，固氮作用不重要，但最近的数据表明，UCYN-A存在多个亚支。UCYN-A亚枝在沿海/近岸环境中对N2的固定分布和速率是海洋N循环中的一个主要未知数。它在近岸水域的存在可能会改变流域氮源（氮固定）和汇（反硝化）之间的平衡模式。同样，在确定受沿海影响水域的新产量估算时，需要考虑到UCYN-A对N2的显著固定。本项目旨在量化不同ucn - a亚系对沿海/近岸N预算的重要性。通过在北美西海岸进行严格的实地考察，解决了在沿海水域中不同UCYN-A亚支系对N2固定率的确定问题。UCYN-A亚枝的时空分布，以及固氮速率，将通过对整体群落和单个细胞（nanoSIMS）的固氮测量与分子分析相结合来确定，以研究这些广泛分布但稀释的重氮营养共生体及其宿主。此外，研究人员还将开展旨在限制光照和养分比（N/P）对ucn - a - N2固定率和原体寄主碳固定率的影响的实验。他们将在南加州海湾（斯克里普斯码头）的一个沿海地点进行季节性采样，并在加利福尼亚中部和下哈半岛之间的沿海水域进行两次过程巡航。巡航工作将提供一个机会来了解在更大的空间尺度上UCYN-A/prymnesiophyte关联的时间动态。最后，有证据表明存在未知的UCYN-A亚枝和宿主，研究人员已经制定了一种策略来确定和量化它们的时空分布以及它们的N2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的固定氮的贡献至关重要。本研究在受沿海影响的水域中测量的群体特定氮固定率和整体氮固定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.