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

氮是一种营养素，其可用性限制了生态系统的生长和生产力。氮在大气中以气态N2的惰性形式非常丰富，但大多数生物不能将N2还原成生物可用的形式。在所有环境中，包括农业土壤，都有微生物可以通过将气态N2还原为生物可利用的形式，即铵，从而从气态N2中获得氮。在广阔的海洋中，很少有生物具有这种能力，最近发现了单细胞蓝藻和单细胞藻类之间的独特共生关系，这似乎分布非常广泛，可能具有全球生物地球化学意义。在这种共生关系中的蓝细菌具有非常不寻常的代谢和基因组精简。很少有人知道的共生，因为它是无法检测，除非通过现代分子生物学技术。最近的研究表明，这种共生现象在海洋中广泛传播，而且在蓝藻和藻类宿主中存在着以前未被认识到的多样性。本研究将探讨美国沿海沃茨的环境分布和这种多样性的地球化学意义。调查人员将通过在当地高中和斯坦福大学的本科生实习项目，并通过在“虚拟巡航”博客中记录他们的实地研究来吸引公众参与海洋科学。在海洋环境中，N2固定对固定氮（N）库的贡献很难量化，部分原因是对固氮生物的丰度，活性和生理学的不完全理解。共生单细胞蓝细菌（UCYN-A）是一个缺乏特征的，但全球重要的海洋固氮生物群。UCYN-A广泛分布于海洋环境中，与一种超微型真核原生植物共生。我们现在知道，有多种生态型的UCYN-A，这可能是适应特定的位置在水柱和不同的海洋省份。通常，N2固定被认为是不重要的沿海影响和非热带沃茨，但最近的数据显示，UCYN-A的多个亚支存在。UCYN-A亚支在沿海/近岸环境中的N2固定的分布和速率是海洋N循环中的一个主要未知数。它在近岸沃茨的存在可能会改变流域氮源（N2固定）和汇（反硝化）之间的平衡模式。 同样，在确定受沿海影响的沃茨的新产量估计数时，需要考虑UCYN-A的显著固氮作用。该项目旨在量化不同UCYN-A亚支对沿海/近岸氮预算的重要性。它解决的问题，确定N2固定率不同UCYN-A亚分支在沿海沃茨通过严格的实地考察关闭北美西海岸。UCYN-A亚分支的时空分布，以及N2固定率，将通过耦合N2固定测量散装社区和单个细胞（nanoSIMS）与分子测定来研究这些广泛的，但稀释，重氮共生体和它们的主机。此外，研究人员还将进行旨在限制光照和营养比（N/P）对UCYN-A N2固定率和原生植物宿主的碳固定率的影响的实验。他们将通过对南加州湾（斯克里普斯码头）的一个沿海地点进行季节性采样以及在加州中部和下半岛之间的沿海沃茨进行两次工艺巡航来开展这项工作。 巡航工作将提供一个机会，了解时间动态的UCYN-A/prymnesiophyte协会在更大的空间尺度。最后，有证据表明，身份不明的UCYN-A亚支和主机存在，研究人员已经制定了一项战略，以确定和量化其时空分布以及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其对海洋环境固定氮的贡献至关重要。在这项研究中测量的沿海影响的沃茨的特定组和散装率的N2固定，将提供数据，为未来的建模工作，这将作出重要贡献，以限制海洋N2固定输入。

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.