A novel hydrogen isotope proxy for trophic behaviour in phytoplankton

海洋初级生产的固碳提供了大量的碳汇，通过生物泵负责每年约7.2 Pg C的固碳。传统的范式假设严格区分自养浮游植物和异养浮游动物，因此，所有初级生产都可以严格归因于自养浮游植物。然而，最近的研究表明，混合浮游生物（原生生物和细菌，既能光养又能吞噬）构成了全球浮游生物群落的重要组成部分。这些混合营养物显著地改变了海洋食物网的结构，增加了生物质的向食物链转移的速度，最近的模型表明，这可能最终将生物泵的效率提高35%。这一发现对从气候模拟到渔业管理等领域具有重大意义。然而，为了充分研究混合营养对全球生物地球化学循环和海洋食物网的影响，将需要可靠的技术来估计混合营养放牧。目前测量吞噬细胞放牧的方法无法区分混合营养体和异养体的贡献，而测量初级生产的方法无法区分混合营养体和纯光养体。迄今为止最成功的技术是利用荧光标记的细菌或藻类。然而，这些技术通常需要杀死猎物，一些原生物种在放牧时避开惰性颗粒。这种行为在捕食与自己体型相似的生物的物种中更为常见，这为检查混合营养的影响提出了问题。一些主要的藻类食草动物，如鞭毛藻，对惰性猎物有积极的歧视。使用活的荧光猎物也有困难。这些方法需要通过引入荧光标记的猎物生物来改变观察到的系统，从而人为地增加猎物的丰度。此外，食草动物对有标记的猎物没有选择性。另一种方法是使用酸性营养探针对食物液泡进行染色，但这需要流式细胞术，无法检测到较大的食草动物，如纤毛虫和鞭毛虫。最终，所有目前使用的方法都需要事先了解该系统，以便确定其中浮游生物的营养行为。目前，还没有能够同时重建浮游生物异养生长时空变化的方法。这个项目将开发一种新的工具，允许在一系列浮游生物群体中进行这种重建。我们将利用特定浮游生物脂质的氢同位素特征来测量浮游生物在变化条件下发生的代谢变化。由于之前在自养、混合养和异养条件下的培养，以及它们产生甲藻特异性脂质、甲藻甾醇的能力，甲藻Micans将被用作氢代谢分离的模型。这种生物标记物的使用将使我们能够跟踪鞭毛藻在当前海洋和整个地质时期的代谢行为，提供这些原生生物代谢行为的空间和时间分辨率。这种方法将应用于其他群体，如原体和硅藻，使用烯酮和油菜甾醇作为各自的生物标志物。

Carbon fixation by oceanic primary production provides a substantial sink for carbon, responsible for the sequestration of about 7.2 Pg C yr via the biological pump. Traditional paradigms assume a strict distinction between autotrophic phytoplankton and heterotrophic zooplankton, and consequently, that all primary production can be attributed to strictly autotrophic phytoplankton. However, recent research has shown that mixoplankton (protists and bacteria capable of both phototrophy and phagotrophy) comprise a substantial part of the global plankton community. These mixotrophs significantly alter the structure of marine food webs and increase the rate of biomass transfer up the food chain, which recent modelling suggests may ultimately enhance the efficiency of the biological pump by up to 35%. This discovery has major implications for fields ranging from climate modelling to the management of fisheries.However, to fully investigate the impact of mixotrophy upon global biogeochemical cycles and marine food webs robust techniques to estimate mixotrophic grazing will be required. Currently methods which measure grazing by phagotrophs are unable to distinguish between mixotroph and heterotroph contributions, while those that measure primary production are unable to distinguish between mixotrophs and pure phototrophs.The most successful techniques to date have employed fluorescently labelled bacteria or algae. However, these techniques often require the prey to be killed, and some protist species avoid inert particles when grazing. This behaviour is more frequent in species which prey on organisms similar to them in size, which presents problems for examining the impacts of mixotrophy. as some major algal grazers, like dinoflagellates, actively discriminate against inert prey.The use of live fluorescent prey also presents difficulties. These methods require the observed system to be altered through the introduction of fluorescently labelled prey organisms, resulting in the artificial increase in prey abundance. Additionally, grazers are not selective towards or against the labelled prey. An alternative method is to use acidotrophic probes to stain food vacuoles, but this requires flow cytometry, which can fail to detect larger grazers such as ciliates and dinoflagellates. Ultimately, all currently used methods require prior knowledge of the system in order to determine the trophic behaviour of plankton within it.Currently, no methods capable of reconstructing both the spatial and temporal variations in heterotrophic growth of microplankton exist. This project will develop a novel tool to allow this reconstruction across a range of plankton groups.We will use the hydrogen isotope signature of specific plankton lipids to measure the metabolic changes occurring in plankton in response to changing conditions. The dinoflagellate P. Micans will be used as a model for the metabolic fractionation of hydrogen due to its previous cultivation under autotrophic, mixotrophic and heterotrophic conditions, and their ability to produce the dinoflagellate-specific lipid, dinosterol. The use of this biomarker will allow us to track the metabolic behaviour of dinoflagellates across the current oceans and throughout geological time, providing both spatial and temporal resolution of the metabolic behaviour of these protists. This approach will be applied to other groups such as prymnesiophytes and diatoms, using alkenones and brassicasterol as the respective biomarkers.