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。虽然许多生物体使用地磁场进行导航是众所周知的，但到目前为止，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触发进化过程，都是完全未知的。最古老、最简单、研究最充分的磁敏生物是趋磁细菌（MTB），它也是唯一一种磁传感器被明确确定为细胞内磁晶体（磁小体）的高度有序链的生物。这些独特的细胞器引导MTB沿着地磁场线的游泳运动，以促进细胞在化学分层的水生沉积物中朝向其优选的亚氧栖息地的导航。MTB的磁小体被认为是作为具有诊断古地磁特性的磁化石保存下来的。然而，它们从死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却知之甚少。在这个项目中，我们建议研究MTB的微进化适应磁小体生物矿化和其固有的游泳极性，在高度控制的实验室条件下模拟地磁场逆转。使用alphaproteobacterium Magnetoelllum gryphiswaldense作为一个模型，我们将进行一个微进化实验，通过连续培养叠加在垂直氧化还原梯度与定义的磁场的可变强度和方向。预期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的自发突变体将通过基因组重测序、鉴定的靶点的定向缺失以及通过显微镜和生物物理方法在遗传和细胞水平上进行彻底分析，以表征其运动性和磁性。此外，各种先前产生的突变体与不同的强度和配置的磁传感器，以及突变体与受损的化学传感系统将进行研究，以模拟不同的假定阶段的进化适应。在DeepDyn优先计划中计划的几项合作中，这些突变体沿着未修饰的野生型M. gryphiswaldense也将在实验室规模的实验中表征磁小体的磁性化，以及噬菌原生动物对磁小体的啃食和随后的溶解。预期的研究结果不仅将揭示趋磁性在当前地磁场中的适应功能，而且将有助于阐明古山地车对过去地磁场变化的进化适应，并将其作为磁化石保存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.