Tracing the food chain from magnetotactic bacteria to protozoa under laboratory and environmental conditions

趋磁细菌（MTB）产生的磁小体化石是古地磁研究的重要代表，用于重建地球历史上过去地质事件的时间。然而，在自然环境中控制结核分枝杆菌丰度的环境和生态过程及其磁小体尚不清楚。结核分枝杆菌生态学的一个关键方面仍然缺乏特征，即它们在自然环境中的丰度如何受到微生物捕食者：掠食性原生动物（原生动物）的控制。我们将测试关于原生生物放牧是否在控制自然界中MTB丰度中起作用以及原生生物放牧对磁小体保存的影响的假设。为了应对这一挑战，我建议采用一种新的实验装置，首次直接确定在环境条件下消耗MTB的原生生物的全部多样性，并揭示在受控的实验室条件下MTB的放牧和磁小体溶解率。我的实验室开发了一种新的实验装置，可以识别以同位素标记的猎物细胞为食的原生生物，我们将把它应用于自然环境，以揭示在海洋和淡水环境中捕食MTB的原生生物的多样性。利用一个具有氧-缺氧梯度的受控实验室设置，我们将对突变型和野生型MTB进行原生生物放牧实验，以确定哪种MTB表型在帮助MTB逃脱某些原生生物的捕食中发挥作用。这将与Dirk sch<e:1>博士（拜罗伊特大学）教授合作完成，他拥有大量具有不同表型的突变MTB菌株的培养集合，我们将在我们的原生放牧实验中应用。这将揭示MTB在主要原生捕食者存在时对其生存至关重要的表型，以及它们如何在特定环境条件下避免捕食。我们还将评估原生生物对磁小体的溶解率，原生生物在吞下MTB后可以溶解磁小体。对于那些被摄入，但没有完全溶解的磁小体，我们将评估磁小体形态和磁性质的变化。与Prof. Dr. Stuart Gilder （LMU Munich）合作，通过在放牧实验结束时对摄入的磁小体进行一阶反转曲线（FORC）分析，研究摄入的磁小体磁性的改变。这将揭示原生生物放牧如何影响磁小体的磁性，这是磁小体石化过程中一个重要但鲜为人知的因素。由于MTB产生的磁小体在过去地质事件的古地磁研究中是一个重要的代理，因此该项目的结果将有助于解释地质记录中的磁化石。

Fossilized magnetosomes produced by magnetotactic bacteria (MTB) are a crucial proxy in paleomagnetism studies, that are used to reconstruct the timing of past geological events in Earth history. However, the environmental and ecological processes controlling the abundance of MTB in natural environments, and their magnetosomes, are poorly understood. One key aspect of MTB ecology that remains poorly characterized, is how their abundance in natural environments is controlled by microbial predators: predatory protozoa (protists). We will test hypotheses surrounding whether protist grazing plays a role in controlling MTB abundance in nature and the effects of protist grazing on the preservation of magneotosomes. I propose to address this challenge by applying a novel experimental setup that will, for the first time, directly identify the full diversity of protists that consume MTB under environmental conditions and reveal rates of MTB grazing and magnetosome dissolution under controlled laboratory conditions. My lab has developed a new experimental setup that allows for identification of protist grazing on isotopically labeled prey cells, that we will apply to natural environmental settings to reveal the diversity of protists that prey on MTB in both marine and freshwater environments. Using a controlled laboratory setup with an oxic-anoxic gradient, we will perform protist grazing experiments with mutant and wild-type MTB to identify which MTB phenotypes play a role in helping the MTB escape predation by certain protists. This will be done in collaboration with Prof. Dr. Dirk Schüler (University of Bayreuth) who maintains a large culture collection of mutant MTB strains with varying phenotypes, that we will apply in our protist grazing experiments. This will reveal the MTB phenotypes that are critical for their survival in the presence of key protist predators, and how they avoid predation under certain environmental conditions. We will also assess rates of magnetosome dissolution by the protists, that can dissolve the magnetosomes after the MTB have been ingested. For those magnetosomes that are ingested, but not completely dissolved, we will assess the changes in magnetosome morphology and magnetic properties. Altered magnetic properties of ingested magnetosomes will be studied in collaboration with Prof. Dr. Stuart Gilder (LMU Munich), by means of first order reversal curve (FORC) analysis of ingested magnetosomes at the end of the grazing experiments. This will reveal how protist grazing effects the magnetic properties of the magnetosomes, an important but poorly understood factor in the magnetosome fossilization process. Because the magnetosomes produced by MTB are a crucial proxy used in paleomagnetism studies of past geological events, the results of this project will aid the interpretation of magnetofossils in the geological record.