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

海洋固氮对海洋生产力和全球营养和碳生物地球化学的重要性已得到普遍认识。由于气候变率和变化，海洋氮固定率和海洋氮储量也被认为在地质时期有所不同。然而，海洋中占优势的固氮菌，如毛菌和单细胞固氮蓝藻，对过去、现在和未来全球大气CO2状态的反应几乎一无所知。我们的初步数据表明，大西洋和太平洋木霉菌分离物的N2和CO2固定率、生长速率和元素比受其生长环境CO2浓度的控制。在预测的2100 pCO2 （750 ppm）年，这两种菌株的N2固定率增加了35-100%，同时C固定率和细胞N:P和C:P比值也增加了。令人惊讶的是，无论生长温度或磷有效性如何，由于CO2升高而导致的N2和C固定的增加都具有相似的相对幅度。因此，二氧化碳的影响似乎独立于其他常见的生长限制因素。同样重要的是，在低二氧化碳分压（150ppm）水平下，木霉属的生长和固氮完全停止，这表明在最后一次冰期最高二氧化碳分压（~ 190ppm）水平下，该属的重氮繁殖可能最多是边缘的。遗传证据表明，重氮三角菌受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。这些发现可能会迫使人们重新评估过去海洋固氮在冰期/间冰期气候变化中的假设作用，并考虑未来高二氧化碳海洋中海洋重氮化作用增加和营养物质和碳循环改变的可能性。我们提出了一个跨学科的项目来研究海洋固氮蓝藻与变化的二氧化碳分压之间的关系。现场和实验室相结合的方法将结合现场测量和实验操作，使用天然和培养的菌群和单细胞N2固定菌，覆盖从冰川时代到未来浓度（150-1500 ppm）的二氧化碳分压范围。我们还将研究二氧化碳分压对N2和C固定和元素化学计量的影响是如何被其他潜在的生长限制变量（如铁、磷、温度和光）的可用性所调节的。我们计划详细了解重要海洋重氮营养体对pCO2变化的全方位响应，包括生长速率、N2和CO2固定、细胞元素比、固定N释放、光合生理以及转录物和蛋白质水平上参与碳氮获取的关键基因的表达。更广泛的影响该项目将为具有所有三个pi的研究生和本科生以及博士后学者提供教育机会。本科和K-12教育将通过参与REU指导，高中毕业论文研究项目和课堂课程开发而进一步发展。计划以一系列关于海洋全球变化问题的公开讲座的形式向公众宣传。这项研究有可能进化我们对海洋中N2固定控制的理解。我们目前关于海洋N2固定、大气CO2、营养生物地球化学、海洋生产力和全球气候变化之间相互作用的许多观点可能需要修正，以考虑到以前未被认识到的大气成分和重氮营养体之间的反馈机制。因此，我们的研究结果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。这个项目将迈出至关重要的第一步，以了解生物地球化学的关键过程，即海洋中N2的固定，如何在未来一个世纪对我们快速变化的世界做出反应。

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.