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（EFA）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的生理功能所必需的。大多数甲壳类动物，包括占优势的浮游动物、桡足类和高等消费者都不能合成它们；因此，他们必须依赖饮食供应。浮游藻类是甾醇和必需脂肪酸的主要生产者，但必需脂肪酸和甾醇的数量和质量在不同藻类中存在显着差异。缺乏必需脂肪酸和/或甾醇的藻类是较差的食物，会导致桡足类生长缓慢或繁殖产量低。在非花期，微型浮游动物，特别是异养原生生物，构成了浮游动物饮食的主要部分。有充分证据表明，异养原生生物（异养原生生物）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游的营养供应，使得捕食异养原生生物的桡足类获得高生长和繁殖成功率。这种“营养升级”效应归因于所谓的异养原生生物将EFA和膳食甾醇的藻类脂肪酸前体修饰成其他形式和/或从头合成EFA和甾醇的能力。尽管越来越多的证据表明一些异养原生生物可以升级浮游动物的必需营养物质，但其生化机制仍然未知。为了充分了解浮游食物网内的必需营养动态，需要进行详细的生化过程研究。该项目的目标是阐明几种常见异养甲藻和纤毛虫中 EFA 和甾醇代谢（同化、转化和从头合成）的生化过程。将采用先进的细胞、生化、微胶囊和稳定同位素技术来研究这些海洋浮游异养原生生物中EFA和甾醇的代谢和相关生化途径。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)异养原生生物从头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、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现场研究奠定基础。详细的生化研究还可能揭示抗议物种之间重要的生物学相似性和差异，否则这些差异很难辨别。学术价值：目前对浮游动物营养的理解大多局限于生物体水平，并且通常基于淡水系统的研究；然而，海洋浮游食物网的营养组织和动态与淡水食物网有很大不同。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，并提供有关海洋系统生产者-消费者界面的细胞和分子水平的基本营养动态的重要信息。更广泛的影响：该项目将为年轻科学家（研究生、本科生和博士后）提供假设建立和测试的机会，并接受海洋生物化学、分子生物学和浮游生物生态学高级研究方法的实践培训。除了培训研究生、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。结果将通过会议演讲和出版物向科学界传播，并通过研究所正在进行的外展活动向公众传播。

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.