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

海洋固定氮气对当前海洋生产力和全球营养和碳生物地球化学的重要性现已得到普遍认可。由于气候变化和变化，海洋氮气固定率和海洋氮素库存也被认为在地质时间上有所不同。然而，对于海洋中主要的氮气固定物，如毛霉和单细胞氮气固定蓝藻对过去、现在和未来全球大气二氧化碳状况的响应，几乎一无所知。我们的初步数据表明，大西洋和太平洋毛霉菌的固定氮气和二氧化碳的速率、生长速率和元素比率受到它们生长所处的环境二氧化碳浓度的控制。在预计的2100年二氧化碳分压(750ppm)时，两个菌株的氮气固定率增加了35%-100%，同时碳固定率和细胞N：P和C：P比率也增加了。令人惊讶的是，无论生长温度或磷的有效性如何，由于二氧化碳升高而导致的氮和碳固定的增加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增长限制因素。同样重要的是，在低二氧化碳水平(150ppm)下，毛霉菌的生长和固氮完全停止，这表明该属的重氮营养作用可能在最好的冰川最大二氧化碳水平~190ppm时是边缘的。遗传证据表明，重氮营养毛霉受到二氧化碳的控制，因为这种蓝藻缺乏高亲和力的溶解无机碳运输能力。这些发现可能迫使重新评估过去海洋固定氮在冰川/间冰期气候变化中的假设作用，以及考虑未来高二氧化碳海洋中增加海洋重氮营养和改变营养和碳循环的可能性。我们提出了一个跨学科的项目来研究海洋中固定蓝藻的氮气与二氧化碳变化之间的关系。现场和实验室相结合的方法将结合现场测量和实验操作，使用自然和培养的毛霉菌和单细胞氮气固定剂，范围从冰河时代的二氧化碳到未来的浓度(150-1500ppm)。我们还将研究二氧化碳对氮和碳的固定以及元素化学计量比的影响如何被其他潜在的生长限制变量的可用性所缓和，如铁、磷、温度和光。我们计划在转录本和蛋白质水平上详细了解重要的海洋重氮菌对二氧化碳变化的反应，包括生长速度、氮和二氧化碳的固定、细胞元素比率、固定氮的释放、光合作用生理以及涉及碳和氮获取的关键基因的表达。广泛的影响这个项目将为所有三个PIs的研究生和本科生以及博士后学者提供教育机会。本科生和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来进一步发展。计划以一系列关于海洋全球变化问题的公开讲座的形式进行公众宣传。这项研究有可能改变我们对海洋中氮气固定的控制机制的理解。我们目前对海洋固定氮、大气二氧化碳、营养生物地球化学、海洋生产力和全球气候变化之间的相互作用的许多想法可能需要修订，以考虑到以前未知的大气成分和重氮菌之间的反馈机制。因此，我们的发现可能会对人类社会及其在不确定的未来对海洋资源日益增长的依赖产生重大影响。该项目将迈出至关重要的第一步，以了解生物地球化学关键过程--海洋中固定氮气--可能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.