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有着深远的影响。浮游植物的生长速率和分布反映了营养盐和光照等生长限制因子的分布。了解影响浮游植物生物量和活动的环境因子是生物海洋学的首要目标。这个问题已经从许多方向（包括瓶富集研究，分子探针和原位营养富集），每种技术都有独特的优点和缺点。最近在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常量营养素生长条件。由于荧光特征确实引人注目，并且可以在不需要孵育或富集的情况下进行评估，因此它们提供了一种途径，以更传统的方法无法达到的规模来调查天然浮游植物种群中的营养限制。诊断荧光特性的解决，使用超过11年的实地测量，涉及昼夜变化的归一化可变荧光，光系统II的功能吸收截面，和电子周转时间的质体醌池。一个潜在的机制基础的诊断已经提出了基于几十年的定向实验室研究铁胁迫对浮游植物的影响，但一个突出的问题仍然存在：推断的机制从来没有在现场证明和诊断昼夜荧光模式从来没有在实验室重现。缺乏对所涉及的生理机制的确认（以及因此它们对特定营养条件的特异性）破坏了诊断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中生理过程和表达之间的差距。所提出的方法包括现场和实验室组成部分，都旨在将昼夜荧光特性与拟议（或修订）机制联系起来。NOAA TAO计划免费为该项目提供现场组件的运输时间。该项目的智力价值在于（1）提高了对天然海洋浮游植物种群中铁胁迫效应及其对测量的光合效率的影响的理解，（2）在实验室中解决与可变荧光属性相关的特定细胞应激反应，以及（3）如果成功，建立一种新的办法，在真正的海盆规模上评估公海的营养物质压力。更广泛的影响：更广泛的影响包括：（1）如果拟议的荧光诊断机制正确，HNLC区域对全球海洋生产的贡献可能会发生重大调整，（2）使用遥感天然荧光全球监测铁胁迫浮游植物种群分布和变化的方法，（3）对更好地解释野外收集的可变荧光数据作出了重要贡献。该项目将有助于培养一名博士后研究员，并将尽一切努力招聘合格的少数民族担任这一职务。我们将鼓励被选中的员工定期参加专业会议和社会活动。此外，还将继续开展公共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.