Microevolutionary adaptations of magnetotactic bacteria to reversals of the geomagnetic field

生命是在地球磁场的存在下进化的。虽然众所周知，许多生物利用地磁场进行导航，但到目前为止，人们完全不知道它们如何适应地球磁偶极子的逆转，以及这些变化是否以及如何引发进化过程。最古老、最简单、研究最深入的生物是趋磁细菌(MTB)，它们也是唯一明确地将磁传感器确定为细胞内高度有序的磁性晶体链的生物，即磁小体。这些独特的细胞器引导着结核杆菌沿着地磁线的游动，以便于细胞在化学分层的水生沉积物中导航到它们偏爱的亚缺氧栖息地。来自MTB的磁小体被认为是保存下来的具有诊断古地磁性质的磁化石。然而，它们从死亡细胞中释放后的命运，其他生物捕食的影响，以及有利于磁小体保存的地球化学条件，人们知之甚少。在这个项目中，我们计划在高度可控的实验室条件下，研究MTB关于磁小体生物矿化的微进化适应以及它们固有的游泳极性对模拟的地磁场逆转的适应。以甲蛋白细菌为模型，我们将在垂直氧化还原梯度叠加不同强度和方向的磁场的情况下，通过连续培养进行微进化实验。自发突变预计将表现出改变的趋磁游泳行为和极性，以及可能的磁小体生物合成，将通过基因组重测序、定向删除已识别的靶标以及显微和生物物理方法在遗传和细胞水平上进行彻底分析，以表征它们的运动性和磁性。此外，还将研究各种先前产生的具有不同强度和磁传感器配置的突变体，以及化学感觉系统受损的突变体，以模拟进化适应的不同假设阶段。在DeepDyn优先计划内计划的几个合作中，还将在实验室规模的实验中表征这些突变体以及未经修改的格雷菲斯瓦尔登斯分枝杆菌野生型细胞，以研究磁小体的石化，以及食菌原虫对磁小体的捕食和随后的溶解。预期的发现不仅将揭示现今地磁场中趋磁性的适应功能，还将有助于阐明古代MTB对过去地磁场变化的进化适应，并将它们作为磁性化石保存下来的情况与地磁场记录相关联。

Life has evolved in the presence of the Earth’s magnetic field. While it is well established that many organisms use the geomagnetic field for navigation, it is entirely unknown so far how they adapt to reversals of the Earth’s magnetic dipole, and whether and how these changes may trigger evolutionary processes. The most ancient, simplest, and best studied magnetosensitive organisms are magnetotactic bacteria (MTB), which are also the only organisms for which the magnetic sensor has been unambiguously identified as highly ordered chains of intracellular magnetic crystals, the magnetosomes. These unique organelles direct the swimming motility of MTB along the geomagnetic field lines, in order to facilitate the cells' navigation towards their preferred suboxic habitats within chemically stratified aquatic sediments. Magnetosomes from MTB are thought to become preserved as magnetofossils with diagnostic palaeomagnetic properties. However, their fate after release from dead cells, the influence of predation by other organisms, and the geochemical conditions favoring magnetosome preservation are poorly understood. In this project, we propose to study microevolutionary adaptations of MTB with respect to magnetosome biomineralization and their inherent swimming polarity to simulated geomagnetic field reversals under highly controlled experimental lab conditions. Using the alphaproteobacterium Magnetospirillum gryphiswaldense as a model, we will perform a microevolution experiment by serial cultivation within vertical redox gradients superimposed with defined magnetic fields of variable intensity and directions. Spontaneous mutants which are expected to exhibit altered magnetotactic swimming behavior and polarity, and possibly, magnetosome biosynthesis, will be thoroughly analyzed at the genetic and cellular level by genome resequencing, directed deletions of identified targets, as well as by microscopic and biophysical methods to characterize their motility and magnetic properties. In addition, various previously generated mutants with different strengths and configurations of their magnetic sensors, as well as mutants with an impaired chemosensory system will be studied, to mimic different putative stages of evolutionary adaptations. In several collaborations planned within the DeepDyn priority program, these mutants along with unmodified wildtype cells of M. gryphiswaldense will also be characterized in lab-scale experiments on fossilization of magnetosomes, as well as on grazing and subsequent dissolution of magnetosomes by bacterivorous protozoa. Expected findings will not only reveal the adaptive function of magnetotaxis within the current geomagnetic field, but also help to elucidate evolutionary adaptations of ancient MTB to past variations of the geomagnetic field, and to correlate their preservation as magnetofossils with the geomagnetic field record.