Fatty Acid and Sterol Metabolism in Planktonic Heterotrophic Protists: Essential Nutrient Upgrading and Transfer and their Ecological Implications in Pelagic Food Webs

浮游植物-浮游动物界面是海洋食物网中能量和物质流动的重要环节。浮游动物生产的营养限制是调节鱼类补充的关键因素之一。必需脂肪酸（EFAs）和甾醇是远洋生态系统中潜在的限制性营养物质，因为它们是繁殖和广泛生理功能所必需的。大多数甲壳类动物，包括占主导地位的浮游动物、类鱿鱼类桡足类和高等食性动物，都不能合成它们；因此，它们必须依靠膳食供应。浮游藻类是甾醇和脂肪酸的主要生产者，然而必需脂肪酸和甾醇的数量和质量在不同藻类群体之间可能有很大差异。缺乏EFAs和/或甾醇的藻类是不良食物，会导致类花青素桡足类动物生长缓慢或繁殖能力低下。在非水华期，微型浮游动物，特别是异养原生生物，是浮游动物饮食的主要组成部分。异养原生生物（heterotrophic protists）是浮游植物和浮游动物之间重要的营养中介。在缺乏必需营养素的藻类上生长的异养原生生物可以改善食物链上的营养供应，从而使捕食异养原生生物的桡足类获得高生长和繁殖成功。这种“营养升级”效应被认为是由于异养原生生物能够将藻类脂肪酸前体转化为其他形式的脂肪酸和膳食甾醇，以及/或重新合成脂肪酸和甾醇。尽管越来越多的证据表明，一些异养原生生物可以为浮游动物提供必需的营养物质，但其生化机制尚不清楚。为了充分了解浮游食物网中基本的营养动态，有必要对生物化学过程进行详细的研究。本项目的目的是阐明几种常见异养鞭毛虫和纤毛虫的脂肪酸和甾醇代谢（同化、转化和重新合成）的生化过程。先进的细胞、生化、微胶囊化和稳定同位素技术将被用于研究这些海洋浮游异养原生生物中脂肪酸和甾醇的代谢和相关的生化途径。研究人员将研究(1)异养原生生物对藻类脂肪酸和甾醇的摄取动力学和代谢；(2)异养原生生物重新合成必需脂肪酸和甾醇；(3)异养原生生物对藻类脂肪酸和甾醇前体的生化转化。这种全面而系统的方法将为异养原生生物如何影响浮游食物网中的营养环境提供答案，并为未来的实验室和实地研究奠定基础。详细的生物化学研究也可能揭示抗议物种之间重要的生物学相似性和差异性，否则这些相似性和差异性就不那么明显了。知识优势：目前对浮游动物营养的了解主要局限于有机体水平，并且通常基于淡水系统的研究；然而，海洋浮游生物食物网的营养组织和动态与淡水食物网大不相同。拟议的研究将填补传统营养限制研究中未解决的重要空白，并在海洋系统中生产者-消费者界面的细胞和分子水平上提供基本营养动力学的重要信息。更广泛的影响：该项目将为青年科学家（研究生、本科生和博士后）提供建立和检验假设的机会，并在海洋生物化学、分子生物学和浮游生物生态学的先进研究方法方面接受实践培训。除了培养研究生、本科生和博士后外，主要研究人员还致力于通过该项目教育代表性不足的学生。研究结果将通过会议报告和出版物向科学界传播，并通过研究所正在进行的外联活动向公众传播。

The phytoplankton-zooplankton interface is an important link for energy and material fluxes in marine food webs. Nutrient limitation of zooplankton production is one of the key factors regulating fish recruitment. Essential fatty acids (EFAs) and sterols are potential limiting nutrients in pelagic ecosystems because they are required for reproduction and a wide range of physiological functions. Most crustaceans including the dominant zooplankton, calanoid copepods, and higher consumers cannot synthesize them; thus, they must rely on dietary supply. Planktonic algae are the main producers of sterols and EFAs, yet the quantity and quality of essential fatty acids and sterols can vary significantly among algal groups. Algae that are deficient in EFAs and/or sterols are poor food and result in slow growth or low reproductive output by calanoid copepods. In non-bloom periods, microzooplankton, particularly heterotrophic protists, contribute to a major portion of a zooplankter's diet. It is well documented that heterotrophic protists (heterotrophic protists) are prominent trophic intermediaries between phytoplankton and zooplankton. Heterotrophic protists that grow on algae deficient in essential nutrients may improve the nutritional supply up the food chain such that copepods preying on the heterotrophic protists attain high growth and reproductive success. This "trophic upgrading" effect has been attributed to the alleged ability of heterotrophic protists to modify algal fatty acid precursors to EFA and dietary sterols to other forms, and/or de novo synthesize of EFA and sterols. Despite the mounting evidence that some heterotrophic protists can upgrade essential nutrients for zooplankton, the biochemical mechanism(s) remain unknown. To fully understand essential nutrient dynamics within the planktonic food web, detailed biochemical process study is warranted. The goal of this project is to elucidate the biochemical processes of EFA and sterol metabolism (assimilation, transformation and de novo synthesis) in several common heterotrophic dinoflagellates and ciliates. Advanced cellular, biochemical, microencapsulation and stable isotope techniques will be employed to investigate the metabolism and associated biochemical pathways for EFA and sterols in these marine planktonic heterotrophic protists. The investigators will investigate (1) the uptake kinetics and metabolism of algal fatty acids and sterols by heterotrophic protists; (2) de novo synthesis of essential fatty acids and sterols by heterotrophic protists; (3) biochemical transformation of algal fatty acid and sterol precursors by heterotrophic protists. This comprehensive and systematic approach will provide answers as to how heterotrophic protists affect the nutritional environment in the planktonic food web, and lay fundamental groundwork for future lab and field study. Detailed biochemical study may also reveal important biological similarities and differences among protest species that are otherwise less discernible. Intellectual merits: Current understanding of zooplankton nutrition is mostly limited to the organismal level and is often based on studies in freshwater systems; yet the trophic organization and dynamic of the marine planktonic food web is vastly different from a freshwater one. The proposed study will fill an important void not addressed in traditional nutrient limitation studies, and provide important information on essential nutrient dynamics at the cellular and molecular levels across the producer-consumer interface in marine systems.Broader impacts: This project will provide opportunities for young scientists (graduates, undergraduates and postdoc) in hypothesis building and testing and to receive hands-on training in advanced research methods in marine biochemistry, molecular biology and plankton ecology. In addition to training graduates, undergraduates, and post-docs, the principal investigators are committed to educating underrepresented students through this project. Results will be disseminated to the scientific community via conference presentations and publications, and to the general public via ongoing outreach activities at the institute.