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

智力优势：根据初级生产消耗量的全球平均衡量标准，原生生物消费者在营养水平较低的海洋食物网中的能量消耗和元素循环具有足够的规模，足以极大地改变向较高水平消费者和出口的材料转移的效率。目前，我们对这些微生物食物网的步骤、它们在地区或时间上的变化，或者它们可能会如何被气候变化改变，知之甚少。氨基酸的化合物特定同位素分析(CSIA-AA)为我们提供了一种基于浮游动物营养位置(TP)的微生物食物网结构和营养通量的方法，它是浮游植物直接摄食和通过原生微型浮游动物间接转移的营养通量的时间积分。初步的实验室实验表明，该方法的标准应用，使用标记的苯丙氨酸作为初级生产者基线的代表性来源AA，标记谷氨酸作为营养丰富的指示剂AA，不会对原生动物食草动物产生可测量的营养步骤信号。然而，结果也表明，替代的高周转率氨基酸，丙氨酸，在原生动物和后生动物消费者中强烈丰富，并导致野外采集的标本中中浮游动物的TP估计大大高于基于标记谷氨酸的估计。这项研究项目将检验这样一种假设，即标记的丙氨酸为海洋食物网中的原生生物步骤提供了一个可量化的和一致的营养丰富指数。这项研究将涉及三大要素。首先，将利用恒化器系统进行对照实验室实验，比较以浮游植物为食的纤毛虫和鞭毛类食草动物的代表性系列的丙氨酸与其他氨基酸对15N的富集量，并评估从浮游植物通过原生动物捕食者到悬浮摄食的桡足类动物的两步富集量。其次，将通过CSIA-AA分析从太平洋四个不同生态区域现场采集的中游动物，以测试通过后生动物营养步骤(比较悬浮摄食物种和主要食肉动物)丙氨酸浓缩的转移，并评估在广泛的生态条件下TP指数如何随营养结构的不同而变化。最后，CSIA-AA对大小结构浮游动物总磷的评估将被纳入四个区域(赤道太平洋、亚热带北太平洋、加利福尼亚州洋流和哥斯达黎加圆顶)氮流的反向模型，作为解决和比较各种生态条件下通过微生物食物网的通量的主要限制因素。对中游动物营养位置进行适当校准的CSIA-AA评估将为低层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、海洋生态系统模型验证以及更好地了解低层食物网对海洋渔业的能量限制提供一种新的有价值的方法。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)的指导关系，重点是促进未被充分代表的年轻妇女在海洋生态和海洋学方面的科学兴趣和职业道路。作为斯克里普斯本科生研究奖学金(SURF)和CCE-LTER夏季REU计划的一部分，本科生研究人员将参与相关的实践研究活动。在K-12级别，调查人员将与海洋探索研究所合作，后者为圣地亚哥的城市青年带来科学和教育，以及扩展你的地平线圣地亚哥，这是一个为少数族裔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，以及它的可变性和弹性。这一结果将对全球生物地球化学循环和海洋渔业模型产生重要影响。

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.