Ecology and annual dynamics of Thaumarchaeota-driven freshwater nitrification

硝化作用是氮循环的关键步骤，通过亚硝酸盐将氨转化为硝酸盐。 它可以防止淡水中氨的积累，这对饮用水供应至关重要。氨氧化为亚硝酸盐是硝化反应的限速步骤。氨氧化古菌（AOA）在深海和土壤中维持着大量的种群（分别高达40%和5%的原核生物）。部分基于我们自己的研究，深贫营养淡水湖也出现了类似的情况（Herber等人，2019年）。除了（相对）丰度和多样性调查，我们目前还没有了解这些化能自养微生物在淡水环境中的生态和生态系统服务动态。这与已经获得的海洋和土壤硝化作用的知识形成强烈对比。我们以前的研究表明，在博登湖，这是一个重要的饮用水资源超过400万人，一个单一的AOA类型构成了13-21%的原核微浮游生物在全年的浅水层。氨氧化细菌通常是两个数量级少丰富和comammox细菌没有检测到。这种自然环境提供了一个独特的机会，量化的生态系统功能所施加的反硝化AOA，并在同一时间，将此信息的遗传组成的一个单一的adenotypp。该提案旨在阐明淡水AOA种群的环境控制及其提供的生态系统服务的年度动态。将检验两个假设：（I）AOA驱动的硝化作用和CO2固定的动力学遵循浮游生物作为主要氨源向深沃茨演替的年度循环。(II)AOA种群规模的动态主要由病毒驱动。这两个假设将在两个工作包（WP）中进行测试。在WP 1中，CARD-FISH结合与氨单加氧酶结合的荧光标记将被建立为一种快速监测策略的总与活性部分的反渗透AOA，并应用于每年的周期。将使用15 N-铵和13 C-碳酸氢盐标记结合GC-IRMS，EA-IRMS和nano-SIMS测量，在本体和单细胞水平上更详细地分析全年选定的时间点的AOA驱动的硝化和暗CO2固定率。在WP 2中，将使用FACS分选的AOA细胞的电子显微镜和微型宏基因组学来探索病毒感染率和感染病毒的身份。最终的目标是将淡水AOA及其病毒分离到纯培养物中。与此同时，原生生物食物泡的CARD-FISH将提供对AOA的放牧压力的见解。拟议的项目将是重要的，以了解控制两个生态系统服务的基本原则，由亚热带淡水AOA提供：硝化安全饮用水供应和暗CO2固定总湖泊生产力。

Nitrification is essential step for the nitrogen cycle and converts ammonia to nitrate via nitrite. It prevents build-up of ammonia in freshwaters, which is critical for drinking water supply. Ammonia oxidation to nitrite is the rate limiting step in nitrification. Ammonia-oxidizing Thaumarchaeota (AOA) sustain large populations in the deep ocean and in soils (up to 40% and 5% of prokaryotes, respectively). A similar picture is emerging for deep oligotrophic freshwater lakes as based partially on our own research (Herber et al., 2019). Besides (relative) abundance and diversity surveys, we currently have no understanding on the ecology and ecosystem service dynamics of these chemolithoautotrophic microorganisms in freshwater environments. This stands in strong contrast to knowledge already gained on marine and soil nitrification. Our previous research established that in Lake Constance, which is an important drinking water resource for over 4 million people, a single phylotype of AOA constituted 13-21% of prokaryotic picoplankton in the hypolimnion throughout the year. Ammonia-oxidizing bacteria were typically two orders of magnitude less abundant and comammox bacteria were not detected. This natural setting provides the unique opportunity to quantify the ecosystem functions exerted by planktonic AOA and at the same time to link this information to the genetic make-up of a single phylotyp. This proposal aims to elucidate environmental controls of freshwater AOA populations and the annual dynamics of ecosystem services they provide. Two hypotheses will be tested: (I) Dynamics of AOA-driven nitrification and CO2 fixation follow the yearly cycle of plankton succession as a major ammonia resource to deep waters. (II) Dynamics of the AOA population size are primarily driven by viruses. Both hypotheses will be tested in two work packages (WP). In WP1, CARD-FISH in combination with a fluorescent label binding to ammonia-monooxygenase will be established as a fast monitoring strategy of the total versus active fraction of planktonic AOA and applied to a yearly cycle. Selected time points throughout the year will be analyzed in more detail in respect to AOA-driven nitrification and dark CO2-fixation rates at both the bulk and single-cell level using 15N-ammonium and 13C-bicarbonate labeling coupled to GC-IRMS, EA-IRMS and nano-SIMS measurements. In WP2, electron microscopy and mini-metagenomics of FACS-sorted AOA cells will be used to explore the rate of viral infection and identity of infecting viruses. An ultimate goal is to isolate the planktonic freshwater AOA and its virus into pure culture. In parallel, CARD-FISH of protist food vacuoles will give insights on grazing pressure on AOA. The proposed project will be important to understand the basic principles controlling two ecosystem services provided by planktonic freshwater AOA: nitrification for safe drinking water supply and dark CO2 fixation for total lake productivity.