Resolving the physiological basis for fluorescence diagnostics of iron stress in natural phytoplankton populations

浮游植物构成了几乎所有海洋生态系统的基础，对全球碳和其他营养物质的循环产生了深远的影响。浮游植物的生长速度和分布反映了营养物质和光照等生长限制因素的分布。了解控制浮游植物生物量和活动的环境因素一直是生物海洋学的首要目标。人们从许多方面(包括瓶子富集研究、分子探针和原位营养富集法)研究了这个问题，每种技术都有独特的优点和缺点。最近对热带太平洋的研究发现了一套新的基于荧光的诊断方法，可以检测特定的微量营养素和大量营养素的生长条件。由于荧光特征确实很显著，而且可以在不需要培养或浓缩的情况下进行评估，它们提供了一种方法，可以在更传统的方法无法达到的规模上调查自然浮游植物种群中的营养限制。诊断的荧光性质是通过超过11年的现场测量来解决的，涉及归一化可变荧光的昼夜变化，光系统II的功能吸收截面，以及质苷库的电子周转时间。基于数十年来对浮游植物铁胁迫效应的实验室定向研究，已经提出了诊断的潜在机制基础，但仍然存在一个悬而未决的问题：所推断的机制从未在现场得到证实，诊断荧光模式从未在实验室中重现。缺乏对所涉及的生理机制的确认(因此它们对特定营养条件的特异性)破坏了诊断方法对描述营养生长条件的效用。该项目旨在弥合自然浮游植物种群的生理过程和表达之间的差距。拟议的方法包括现场和实验室组成部分，两者的目的都是将Diel荧光特性与拟议的(或修订的)机制联系起来。现场组件的运输时间由NOAA TAO计划免费提供给项目。该项目的学术价值在于：(1)更好地了解天然海洋浮游植物种群中铁胁迫的影响及其对测量的光合作用效率的影响；(2)在实验室中结合可变的荧光属性解决特定的细胞胁迫反应；(3)如果成功，将建立一种新的方法，在真正的盆地尺度上评估公开海洋的营养盐胁迫。更广泛的影响：更广泛的影响包括：(1)如果所提议的荧光诊断机制是正确的，HNLC区域对全球海洋生产的贡献可能出现重大调整，(2)利用遥感自然荧光对全球监测铁胁迫浮游植物种群分布和变异性的方法，(3)对更好地对实地收集的可变荧光数据进行更好的生理解释的重要贡献。该项目将有助于培养一名博士后，并将尽一切努力招聘一名合格的少数人担任这一职位。被选中的合伙人将被鼓励定期参加专业会议和社会活动。此外，还将继续开展公众宣传和广泛传播成果。

Phytoplankton constitute the base of nearly all ocean ecosystems and have a profound influence on the global cycling of carbon and other nutrients. The growth rate and distribution of phytoplankton reflect the distribution of growth limiting factors such as nutrients and light. Understanding the environmental factors regulating phytoplankton biomass and activity has been a foremost goal in biological oceanography. This problem has been approached from many directions (including bottle enrichment studies, molecular probes, and in situ nutrient enrichments), with each technique having unique advantages and disadvantages. Recent research in the tropical Pacific Ocean has uncovered a new set of fluorescence-based diagnostics that allow detection of particular micronutrient and macronutrient growth conditions. As the fluorescence features are truly striking and can be evaluated without the need for incubations or enrichments, they provide a path to investigate nutrient constraints in natural phytoplankton populations at a scale inaccessible to the more traditional approaches. The diagnostic fluorescence properties were resolved using over 11 years of field measurements and involve diel changes in normalized variable fluorescence, the functional absorption cross section of photosystem II, and the electron turnover time of the plastoquinone pool. An underlying mechanistic basis for the diagnostics has been proposed based on decades of directed laboratory research on iron stress effects in phytoplankton, but an outstanding issue remains: the inferred mechanisms been never demonstrated in the field and the diagnostic diel fluorescence patterns have never been reproduced in the laboratory. The lack of confirmation of the physiological mechanisms involved (and thus their specificity to specific nutrient conditions) undermines the utility of the diagnostics to describe nutrient growth conditions. This project aims at bridging the gap between physiological processes and expressions in natural phytoplankton populations. The proposed approach includes field and laboratory components, both aimed at linking the diel fluorescence characteristics to proposed (or revised) mechanisms. Ship time for the field component is being provided at no cost to the project by the NOAA TAO Program. The intellectual merit of this project resides in (1) an improved understanding of iron-stress effects in natural marine phytoplankton populations and their impacts on measured photosynthetic efficiencies, (2) resolving specific cellular stress responses in the laboratory in association with variable fluorescence attributes, and (3) if successful, the establishment of a new approach for assessing nutrient stress in the open ocean on a truly basin-scale. Broader Impacts: Broader impacts include (1) a potential significant adjustment in the contribution of HNLC regions to global ocean production if the proposed mechanisms for the fluorescence diagnostic are correct, (2) an approach to globally monitor the distribution and variability of iron-stressed phytoplankton populations using remotely-sensed natural fluorescence, (3) an important contribution toward better physiological interpretation of variable fluorescence data collected in the field. This project will contribute to training of a post doctoral fellow and every effort will be made to recruit a qualified minority for the position. The selected associate will be encouraged to regularly participate in professional meetings and society activities. In addition, continued public outreach and broad dissemination of results will be carried out.