Phosphorus Bioavailability and its Effect on the Role of Trichodesmium in Elemental Cycling

智力优势：世界海洋中巨大的少营养环流约占全球海洋环境的60%。曾经被认为是生物沙漠的最近的研究确定，这些地区可能占到海洋有机碳出口总量的一半。在一个面临着描述和预测我们的生态系统在不断变化的环境压力下的行为的社会中，对这些巨大的海洋生物群的彻底了解可以推动我们对能够适应环境变化的海洋生态系统进行量化表示。在这方面，夏威夷海洋时间序列(HOT)计划在过去15年中对北太平洋副热带环流(NPSG)的连续观测和研究为海洋生物地球化学动力学提供了广泛的记录。这个系统的年度循环是由光合作用和呼吸作用过程之间的紧密耦合所主导的，因此生物产生的大部分碳被循环到系统中。相比之下，NPSG的碳净出口主要发生在夏季，这是大型固定氮光自养植物定期繁殖的结果。这些水华事件的一个基本特征是观察到溶解和颗粒N：P和C：P比值的升高，表明在水华事件期间，生物系统向更强烈的磷(P)限制状态转变。虽然脉冲输出事件的发生在这个系统中得到了很好的记录，但驱动伴随的化学计量比转移的生理机制仍然知之甚少。在这一建议中，研究人员确定了三种与生态有关的生理适应，它们可以定量地解释关键的水华形成生物--毛霉--增加生物量和丰度的能力，改变溶解和颗粒池的化学计量比，从而在原本营养匮乏的海洋环境中调节元素的流动和出口的大小。这些适应是：1)溶解有机库的利用，2)内部磷配额的极端变异性和3)浮力控制。考虑到这些生理适应，本研究的目标如下：1)测定天然木霉种群对可溶性反应性磷(SRP)和溶解有机磷(DOP)的吸收和再生速率。2)获得对不同环境条件下P的相对细胞含量和区划的可塑性的可靠估计，并表征P配额的变化如何影响Trichodesum spp对颗粒和溶解有机碳和氮的有机生产。3)验证浮力介导的毛霉菌落垂直迁移促进磷跃层开采和向真光层注入DIP的假说。4)利用上述研究活动得出的结果，评估和模拟不同气候情景(即在水柱层结增加或减少的时期)下，毛藻在元素(C、N和P)通量和调节远洋生态系统结构中的作用。这些目标的实现将通过整合实地和实验室研究部分来实现。更广泛的影响：研究目标的实施将有助于加强现有的研究伙伴关系，并通过数据共享和批判性论述建立更多的合作网络。除了生成适用于相关研究计划工作的补充数据集外，该项目还将直接为热点计划工作做出贡献，以描述NPSG生物群中的营养循环并对其进行建模。更广泛的教育影响包括为项目中的青年科学家提供广泛的跨学科科学培训，提高妇女在科学领域的地位的机会，以及旨在鼓励高中至本科妇女参与科学事业的教育宣传。我们的研究的更广泛的社会效益是加深了对关键生物群在元素循环中的作用的理解，以及对基本海洋栖息地初级和出口生产的调节。这些知识将使我们能够走向一种能够适应环境变化的海洋生态系统的机械模型。

Intellectual Merit: The vast oligotrophic gyres of the world's ocean encompass roughly 60% of the global marine environment. Once thought to be biological deserts, recent research has determined that these regions may account for up to half of the total oceanic organic carbon export. In a society faced with the task of characterizing and predicting the behavior of our ecosystem under the stress of a changing environment, a thorough understanding of these vast marine biomes can move us toward a quantitative representation of the marine ecosystem that can adapt to environmental change. In this respect, the continuous observation and study of the North Pacific subtropical gyre (NPSG) over the past 15 years by the Hawaii Ocean time-series (HOT) program has provided an extensive record of oceanic biogeochemical dynamics. The annual cycle of this system is dominated by tight coupling between the processes of photosynthesis and respiration such that the majority of biologically produced carbon is recycled to the system. In contrast, net export of carbon in the NPSG occurs primarily during summer periods as a result of regular blooms of large, buoyant N2-fixing photoautotrophs. A fundamental trait of these bloom events is the observation of elevated dissolved and particulate N:P and C:P ratios, indicating that the biological system is shifted to a more intensely phosphorus (P) limited state during bloom events. While the occurrence of pulsed export events is well documented in this system, the physiological mechanisms driving the companion stoichiometric diversions remain poorly understood. In this proposal, the investigators have identified three ecologically relevant physiological adaptations, which may quantitatively explain the ability of a key bloom-forming organism, Trichodesmium, to increase in biomass and abundance, alter stoichiometric ratios of dissolved and particulate pools and thus regulate the flow of elements and the magnitude of export in an otherwise nutrient starved marine environment. These adaptations are: 1) utilization of dissolved organic pools, 2) extreme variability of internal P quotas and 3) buoyancy control. With these physiological adaptations in mind, the objectives of this research are as follows: 1) To measure uptake and regeneration rates of soluble reactive P (SRP) and dissolved organic P (DOP) in natural Trichodesmium populations. 2) To obtain robust estimates of the plasticity of the relative cellular content and compartmentalization of P under different environmental conditions and to characterize how changes in P quotas affect organic production of particulate and dissolved organic carbon and nitrogen by Trichodesmium spp. 3) To test the hypothesis that buoyancy-mediated vertical migration of Trichodesmium colonies facilitates mining of the phosphocline and injection of DIP into the euphotic zone. 4) To utilize the results derived from the above research activities to assess and model the role of Trichodesmium in the flux of elements (C, N, and P) and regulation of pelagic ecosystem structure under different climate scenarios (i.e. under increased or decreased periods of water column stratification). Fulfillment of these objectives will be achieved via the integration of field, and laboratory research components. Broader Impacts: Implementation of the research objectives will serve to enhance existing research partnerships and build additional collaborative networks through data sharing and critical discourse. This project will directly contribute to HOT program efforts to characterize and model nutrient cycles in the NPSG biome in addition to generating complementary data sets applicable to the work of related research programs. Broader educational impacts include the provision of extensive interdisciplinary science training for the young scientists included in the project, opportunities for the advancement of women in science and educational outreach aimed at encouraging participation of high school to undergraduate level women in science careers. The broader societal benefits of our research are an increased understanding of role of key biota in elemental cycling and the regulation of primary and export production in a fundamental oceanic habitat. This knowledge will allow us to move toward a mechanistic model of the marine ecosystem that can adapt to environmental change.