Collaborative Research: Dissolved organic phosphorus controls on marine nitrogen fixation and export production

浮游植物在地球碳、氮元素循环中扮演着重要角色。除了阳光，生活在海洋表层水域的浮游植物还需要氮和磷元素才能生长。在冬季，当水柱的垂直稳定性被破坏时，这些营养物质大部分是以无机形式从深水向表面混合而提供的。然而，由于上层水柱强烈的热层化(垂直稳定性)，许多低纬度海洋，45°S45°N，对阳光充足的表层水域的营养输入有限。因此，热带和亚热带浮游植物设计出了获取氮和磷的替代方法。海洋固氮是专门的微生物利用大气中丰富的氮气将元素氮转化为生物可利用的氨的过程。这些固氮浮游植物和许多其他植物也在低纬度海洋中使用有机形式的磷，那里的无机营养物质往往很稀缺。该项目将大大增加对溶解有机氮和溶解有机磷浓度的测量，特别是对目前采样不足的太平洋和印度洋的测量。在数值模型中，海洋表层有机营养物浓度的变化将被用来推断浮游植物利用有机营养物的速率。具体地说，将从综合数据和模型输出中量化生物吸收溶解的有机磷在刺激低纬海洋的海洋固氮和光合作用过程中的作用。该项目将在实验室化学分析技术和海洋生物和化学过程的数值模拟方面培训一名研究生和几名本科生。新的科学知识将通过社交媒体活动与公众分享，并将为下一代全球气候模型的发展提供信息。海洋生物地球化学模型界已经确定，缺乏从全球海洋300米上层测量到的溶解有机氮(DON)，特别是溶解有机磷(DOP)浓度，是我们准确模拟亚热带出口产量和固氮率能力的关键差距。拟议的工作将显著增加海洋DON和DOP浓度测量的全球数据覆盖率，特别是在印度洋、热带南太平洋西部、中部和东部、阿拉斯加湾、东副热带和亚极地南太平洋、南大洋、亚热带北大西洋和热带南大西洋盆地中采样不足的海洋区域。这些新的测量结果将被吸收到最先进的生物地球化学模型中，以限制DOP和DON的相对循环速率，并量化优先DOP消耗作为支持低纬海洋出口生产和氮气固定的磷源的作用。模型输出将解决以DON和DOP累积的新产量的区域解析比例、自养DOP吸收速率以及DON和DOP的再矿化率。模式输出还将包括对维持出口生产的真光区DOP消费和氮固定的第一次区域可变速率估计，以受来自太平洋和印度洋的观测限制。因此，新的浓度测量和诊断模型将使我们能够评估DOP消费和再循环中的区域变异性在支持低纬海洋的出口生产和固定氮气方面的量化作用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Phytoplankton play an important role in the Earth's elemental cycles of carbon and nitrogen. In addition to sunlight, phytoplankton living in the surface waters of the oceans require the elements nitrogen and phosphorus for growth. Much of these nutrients are supplied in their inorganic forms from mixing of deep waters towards the surface during the winter months when vertical stability of the water column breaks down. However much of the low latitude oceans, 45degS45degN, suffer from limited nutrient input to sunlit surface waters due to strong thermal stratification (vertical stability) of the upper water column. As a consequence, tropical and subtropical phytoplankton have devised alternative ways of acquiring nitrogen and phosphorus. Marine nitrogen fixation is a process by which specialized microbes utilize the abundant nitrogen gas from the atmosphere to convert elemental nitrogen into the bioavailable form ammonia. These nitrogen fixing phytoplankton and many others also use organic forms of phosphorus in the low latitude ocean where inorganic nutrients are often scarce. This project will significantly increase the number of dissolved organic nitrogen and dissolved organic phosphorus concentration measurements, especially from the currently under-sampled Pacific and Indian Oceans. Changes in the concentration of organic nutrients across the surface ocean will be used to infer rates of organic nutrient use by phytoplankton in numerical models. Specifically, the role for the biological uptake of dissolved organic phosphorus to stimulate the processes of marine nitrogen fixation and photosynthesis in the low latitude ocean will be quantified from the combined data and model output. The project will train one graduate student and several undergraduate students in both laboratory chemical analysis techniques and numerical simulation of ocean biological and chemical processes. New scientific knowledge will be shared with the public via a social media campaign and will inform the development of the next generation of global climate models. The marine biogeochemical modeling community has identified the lack of dissolved organic nitrogen (DON) and especially dissolved organic phosphorus (DOP) concentration measurements from the upper 300 m of the global ocean as crucial gaps in our ability to accurately model export production and N2 fixation rates in the subtropics. The proposed work will significantly increase global data coverage of marine DON and DOP concentration measurements, in particular from under-sampled ocean regions in the Indian Ocean, western, central, and eastern tropical South Pacific, Gulf of Alaska, eastern subtropical and subpolar South Pacific, Southern Ocean, subtropical North Atlantic, and tropical South Atlantic Ocean basins. These new measurements will be assimilated in state-of-the-art biogeochemical models to constrain the relative cycling rates of DOP and DON and to quantify the role of preferential DOP consumption as a P source supporting export production and N2 fixation in the low latitude ocean. Model output will solve for the regionally-resolved fraction of new production that accumulates as DON and DOP, autotrophic DOP uptake rates, as well as the remineralization rates for DON and DOP. The model output will also include the first regionally variable rate estimates of euphotic zone DOP consumption sustaining export production and N2 fixation to be constrained by observations from the Pacific and Indian Oceans. Thus, the new concentration measurements and diagnostic modeling will allow us to evaluate the quantitative role for regional variability in DOP consumption and recycling that supports export production and N2 fixation in the low latitude ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.