Resolving the trophic connection between protistan grazers and mesozooplankton in marine food webs using amino acid-specific stable isotope analyses

智力优点：能量耗散和元素循环的原生生物消费者在较低的营养水平的海洋食物网是足够的规模，根据全球平均措施的初级生产消费量，强烈改变的效率，材料转移到更高层次的消费者和出口。我们目前对这些微生物食物网步骤知之甚少，它们如何在区域或时间上变化，或者它们如何被气候变化改变。化合物特定的氨基酸同位素分析（CSIA-AA）提供了一种方法，推进我们的理解微生物食物网结构和营养通量的基础上的营养位置（TP）的浮游动物作为时间积分器的通量直接喂养浮游植物和间接转移通过原生微型浮游动物。初步的实验室实验，以测试这一想法已经证明，该方法的标准应用，使用标记的苯丙氨酸作为代表源AA的初级生产者基线和标记的谷氨酸作为指示剂AA的营养富集，不产生可测量的营养级信号原生生物食草动物。然而，结果也表明，替代高营业额的AA，丙氨酸，强烈富集原生动物以及后生动物消费者，并导致显着更高的TP估计浮游动物在现场采集的标本比标记的谷氨酸的基础上。这项研究项目将测试的假设，标记丙氨酸提供了一个可量化的和一致的营养富集指数的原生生物步骤在海洋食物网。这项研究将涉及三个主要内容。首先，控制实验室实验将进行恒化器系统比较15 N富集丙氨酸其他氨基酸的代表性套件的纤毛虫和鞭毛虫食草动物喂养浮游植物，并评估两步富集浮游植物通过原生动物食草动物悬浮液喂养桡足类。第二，现场收集的中型浮游动物从四个不同的生态区域的太平洋将进行分析CSIA-AA测试转移丙氨酸富集通过后生动物的营养步骤（比较悬浮饲养物种的主要食肉动物），并评估如何TP指数不同的营养结构在广泛的生态条件。最后，CSIA-AA评估TP的大小结构的浮游动物将被集成到四个区域（赤道太平洋，亚热带北太平洋，加州电流和哥斯达黎加圆顶）的氮流量的逆模型作为解决和比较通量的微生物食物网的生态条件的范围内的主要限制。一个适当校准的CSIA-AA评估中型浮游动物营养位置将提供一个新的和有价值的方法，较低层次的食物网结构的区域相互比较，评估气候变化的时间和空间趋势，海洋生态系统模型的验证，并更好地了解较低的食物网对海洋渔业的能量限制。更广泛的影响：该项目将涉及各级（博士后、研究生、本科生和K-12）的辅导关系，重点是提高海洋生态学和海洋学领域代表性不足的年轻妇女的科学兴趣和职业道路。本科研究人员将从事动手相关的研究活动，作为斯克里普斯本科研究奖学金（SURF）和CCE-LTER夏季REU计划的一部分。在K-12级别，调查人员将与海洋发现研究所合作，该研究所为圣地亚哥城市青年带来科学和教育，并扩大你的视野圣地亚哥，这是一个为代表性不足的少数民族女孩提供科学活动的项目。这项研究的结果将有助于理解构成食物网基础的机制，其可变性和弹性。研究结果将对全球海洋地球化学循环模式和海洋渔业具有重要意义。

Intellectual merit: Energy dissipation and elemental cycling by protistan consumers in lower trophic levels of ocean food webs are of sufficient magnitude, based on global mean measures of the amount of primary production consumed, to strongly alter the efficiencies of material transfers to higher-level consumers and to export. We presently know very little about these microbial food web steps, how they vary regionally or temporally, or how they might be altered by climate change. Compound Specific Isotope Analysis of Amino Acids (CSIA-AA) offers an approach for advancing our understanding of microbial food web structure and trophic fluxes based on the trophic positions (TP) of mesozooplankton as temporal integrators of the fluxes from direct feeding on phytoplankton and indirect transfers via protistan microzooplankton. Preliminary laboratory experiments to test this idea have demonstrated that the standard application of the method, using labeled phenylalanine as the representative source AA for the primary producer baseline and labeled glutamic acid as the indicator AA for trophic enrichment, does not produce a measureable trophic-step signal for protistan grazers. However, the results have also shown that an alternative high-turnover AA, alanine, strongly enriches in protistan as well as metazoan consumers, and leads to substantially higher TP estimates of mesozooplankton in field-collected specimens than that based on labeled glutamic acid. This research project will test the hypothesis that labeled alanine provides a quantifiable and consistent index of trophic enrichment for protistan steps in marine food webs. The research will involve three major elements. First, controlled laboratory experiments will be conducted with chemostat systems to compare 15N enrichments of alanine to other AAs for a representative suite of ciliate and flagellate grazers feeding on phytoplankton, and to evaluate the two-step enrichment from phytoplankton via a protistan grazer to a suspension-feeding copepod. Second, field-collected mesozooplankton from four distinct ecological regions of the Pacific Ocean will be analyzed by CSIA-AA to test the transfer of alanine enrichment through a metazoan trophic step (comparing suspension feeding species to primary carnivores) and to assess how the TP index differs with trophic structure over a broad range of ecological conditions. Last, CSIA-AA assessments of TP for size-structured zooplankton will be integrated into inverse models of nitrogen flows in the four regions (equatorial Pacific, subtropical North Pacific, California Current and Costa Rica Dome) as a major constraint for resolving and comparing fluxes through the microbial food web over the range of ecological conditions. A properly calibrated CSIA-AA assessment of mesozooplankton trophic position will provide a new and valuable approach for regional intercomparisons of lower-level food web structure, for assessing temporal and spatial trends in climate change, for ocean ecosystem model validation, and for better understanding of lower food-web energetic constraints on ocean fisheries. Broader Impacts: This project will involve mentoring relationships at all levels (postdoctoral, graduate, undergraduate and K-12) with an emphasis on advancing the scientific interests and career paths of young underrepresented women in marine ecology and oceanography. Undergraduate researchers will be engaged in hands-on relevant research activities as part of the Scripps Undergraduate Research Fellowship (SURF) and CCE-LTER summer REU programs. At the K-12 level, the investigators will work with the Ocean Discovery Institute, which brings science and education to San Diego urban youth, and Expanding Your Horizons San Diego, a program that provides scientific activities to girls from underrepresented minorities. Results from the study will contribute to understanding the mechanisms structuring the base of the food web, its variability and resilience. The results will have important implications for models of global biogeochemical cycling and ocean fisheries.