Collaborative Research: CO2 control of oceanic nitrogen fixation and carbon flow through diazotrophs

目前，海洋固氮对海洋生产力和全球营养盐和碳地球化学的重要性已得到普遍承认。海洋N2固定率和海洋氮库存也被认为是由于气候变异和变化而在地质时期发生变化。然而，几乎没有什么是已知的占主导地位的N2固定在海洋中，如束毛藻和单细胞N2固定蓝藻的过去，现在和未来的全球大气CO2制度的反应。我们的初步数据表明，N2和CO2固定率，生长速率，和元素比例的大西洋和太平洋Trichodesmium分离株控制的环境CO2浓度，它们生长。在预测的2100 pCO 2（750 ppm）年，两种菌株的N2固定率增加了35- 100%，同时增加了C固定率和细胞N：P和C：P比。 令人惊讶的是，这些增加N2和C固定由于升高CO2是相似的相对幅度，无论生长温度或P的可用性。因此，二氧化碳的影响似乎与其他常见的生长限制因素无关。 同样重要的是，束毛藻的生长和固氮作用在低pCO 2水平（150 ppm）时完全停止，这表明该属的固氮作用在末次冰期最高pCO 2水平约190 ppm时可能是边缘性的。遗传证据表明，束毛藻固氮是受CO2控制，因为这种蓝藻缺乏高亲和力的溶解无机碳运输能力。这些研究结果可能会迫使重新评估过去的海洋固氮在冰川/间冰期气候变化中的假设作用，以及考虑在未来高CO2海洋中增加海洋固氮和改变营养和碳循环的潜力。 我们提出了一个跨学科的项目来研究海洋N2固定蓝藻和改变pCO 2之间的关系。一个结合现场和实验室的方法将结合原位测量与实验操作，使用自然和培养的群体的束毛藻和单细胞N2固定器的范围内的pCO 2跨越冰川时代的未来浓度（150-1500 ppm）。我们还将研究如何pCO 2对N2和C固定和元素化学计量的影响是由其他潜在的生长限制变量，如铁，磷，温度和光的可用性缓和。我们计划获得重要海洋固氮生物对pCO 2变化的全方位反应的详细图像，包括生长速率，N2和CO2固定，细胞元素比，固定N释放，光合生理，在转录本和蛋白质水平上研究与碳和氮获取有关的关键基因及其表达。更广泛的影响本项目将为研究生和本科生提供教育机会三个私家侦探和一个博士后本科和K-12教育将通过参与REU辅导，高中毕业论文研究项目和课堂课程开发来进一步发展。计划以举办一系列关于海洋全球变化问题的公开讲座的形式开展公众外联活动。 这项研究有可能进化我们对海洋中N2固定控制的理解。我们目前的许多想法海洋固氮，大气CO2，营养盐地球化学，海洋生产力和全球气候变化之间的相互作用可能需要修改，以考虑到以前未被认识到的反馈机制之间的大气成分和固氮生物。因此，我们的发现可能对人类社会及其在不确定的未来对海洋资源的日益依赖产生重大影响。该项目将迈出重要的第一步，以了解海洋中N2的固定这一地球化学关键过程如何应对未来世纪迅速变化的世界。

The importance of marine N2 fixation to present ocean productivity and global nutrient and carbon biogeochemistry is now universally recognized. Marine N2 fixation rates and oceanic N inventories are also thought to have varied over geological time due to climate variability and change. However, almost nothing is known about the responses of dominant N2 fixers in the ocean such as Trichodesmium and unicellular N2 fixing cyanobacteria to past, present and future global atmospheric CO2 regimes. Our preliminary data demonstrate that N2 and CO2 fixation rates, growth rates, and elemental ratios of Atlantic and Pacific Trichodesmium isolates are controlled by the ambient CO2 concentration at which they are grown. At projected year 2100 pCO2 (750 ppm), N2 fixation rates of both strains increased 35-100%, with simultaneous increases in C fixation rates and cellular N:P and C:P ratios. Surprisingly, these increases in N2 and C fixation due to elevated CO2 were of similar relative magnitude regardless of the growth temperature or P availability. Thus, the influence of CO2 appears to be independent of other common growth-limiting factors. Equally important, Trichodesmium growth and N2 fixation were completely halted at low pCO2 levels (150 ppm), suggesting that diazotrophy by this genus may have been marginal at best at last glacial maximum pCO2 levels of ~190 ppm. Genetic evidence indicates that Trichodesmium diazotrophy is subject to CO2 control because this cyanobacterium lacks high-affinity dissolved inorganic carbon transport capabilities. These findings may force a re-evaluation of the hypothesized role of past marine N2 fixation in glacial/interglacial climate changes, as well as consideration of the potential for increased ocean diazotrophy and altered nutrient and carbon cycling in the future high-CO2 ocean. We propose an interdisciplinary project to examine the relationship between ocean N2 fixing cyanobacteria and changing pCO2. A combined field and laboratory approach will incorporate in situ measurements with experimental manipulations using natural and cultured populations of Trichodesmium and unicellular N2 fixers over range of pCO2 spanning glacial era to future concentrations (150-1500 ppm). We will also examine how effects of pCO2 on N2 and C fixation and elemental stoichiometry are moderated by the availability of other potentially growth-limiting variables such as Fe, P, temperature, and light. We plan to obtain a detailed picture of the full range of responses of important oceanic diazotrophs to changing pCO2, including growth rates, N2 and CO2 fixation, cellular elemental ratios, fixed N release, photosynthetic physiology, and expression of key genes involved in carbon and nitrogen acquisition at both the transcript and protein level.Broader ImpactsThis project will provide educational opportunities for graduate and undergraduate students with all three PIs and for a postdoctoral scholar. Undergraduate and K-12 education will be furthered through involvement in REU mentoring, high school senior thesis research projects, and classroom curriculum development. Public outreach is planned in the form of a series of public lectures on ocean global change issues. This research has the potential to evolutionize our understanding of controls on N2 fixation in the ocean. Many of our current ideas about the interactions between oceanic N2 fixation, atmospheric CO2, nutrient biogeochemistry, ocean productivity, and global climate change may need revision to take into account previously unrecognized feedback mechanisms between atmospheric composition and diazotrophs. Our findings could thus have major implications for human society, and its increasing dependence on ocean resources in an uncertain future. This project will take the first vital steps towards understanding how a biogeochemically-critical process, the fixation of N2 in the ocean, may respond to our rapidly changing world during the century to come.